diff --git a/CHANGES.md b/CHANGES.md
index fbd0524..5724687 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,4 +1,5 @@
 - v0.4.4
+    - `unikmer`: add global option `-L/--compression-level`.
     - `unikmer diff`: reduce memory occupation, speed not affected.
 - v0.4.3
     - `unikmer diff`: fix bug of hanging when the first file having no Kmers.
diff --git a/unikmer/cmd/concat.go b/unikmer/cmd/concat.go
index cf86bbc..6f3edf1 100644
--- a/unikmer/cmd/concat.go
+++ b/unikmer/cmd/concat.go
@@ -63,7 +63,7 @@ Attentions:
 		if !isStdout(outFile) {
 			outFile += extDataFile
 		}
-		outfh, gw, w, err := outStream(outFile, opt.Compress)
+		outfh, gw, w, err := outStream(outFile, opt.Compress, opt.CompressionLevel)
 		checkError(err)
 		defer func() {
 			outfh.Flush()
diff --git a/unikmer/cmd/count.go b/unikmer/cmd/count.go
index 69252c3..864cd70 100644
--- a/unikmer/cmd/count.go
+++ b/unikmer/cmd/count.go
@@ -68,7 +68,7 @@ var countCmd = &cobra.Command{
 		if !isStdout(outFile) {
 			outFile += extDataFile
 		}
-		outfh, gw, w, err := outStream(outFile, opt.Compress)
+		outfh, gw, w, err := outStream(outFile, opt.Compress, opt.CompressionLevel)
 		checkError(err)
 		defer func() {
 			outfh.Flush()
diff --git a/unikmer/cmd/diff.go b/unikmer/cmd/diff.go
index e0e82bf..6d1252d 100644
--- a/unikmer/cmd/diff.go
+++ b/unikmer/cmd/diff.go
@@ -103,7 +103,7 @@ Tips:
 				if !isStdout(outFile) {
 					outFile += extDataFile
 				}
-				outfh, gw, w, err := outStream(outFile, opt.Compress)
+				outfh, gw, w, err := outStream(outFile, opt.Compress, opt.CompressionLevel)
 				checkError(err)
 				defer func() {
 					outfh.Flush()
@@ -230,7 +230,7 @@ Tips:
 			if !isStdout(outFile) {
 				outFile += extDataFile
 			}
-			outfh, gw, w, err := outStream(outFile, opt.Compress)
+			outfh, gw, w, err := outStream(outFile, opt.Compress, opt.CompressionLevel)
 			checkError(err)
 			defer func() {
 				outfh.Flush()
@@ -477,7 +477,7 @@ Tips:
 		if !isStdout(outFile) {
 			outFile += extDataFile
 		}
-		outfh, gw, w, err := outStream(outFile, opt.Compress)
+		outfh, gw, w, err := outStream(outFile, opt.Compress, opt.CompressionLevel)
 		checkError(err)
 		defer func() {
 			outfh.Flush()
diff --git a/unikmer/cmd/dump.go b/unikmer/cmd/dump.go
index ab70790..c635e02 100644
--- a/unikmer/cmd/dump.go
+++ b/unikmer/cmd/dump.go
@@ -57,7 +57,7 @@ var dumpCmd = &cobra.Command{
 		if !isStdout(outFile) {
 			outFile += extDataFile
 		}
-		outfh, gw, w, err := outStream(outFile, opt.Compress)
+		outfh, gw, w, err := outStream(outFile, opt.Compress, opt.CompressionLevel)
 		checkError(err)
 		defer func() {
 			outfh.Flush()
diff --git a/unikmer/cmd/grep.go b/unikmer/cmd/grep.go
index 224a6cd..159c333 100644
--- a/unikmer/cmd/grep.go
+++ b/unikmer/cmd/grep.go
@@ -133,7 +133,7 @@ var grepCmd = &cobra.Command{
 			log.Infof("finish reading Kmers from %s", file)
 		}
 
-		outfh, gw, w, err := outStream(outFile, strings.HasSuffix(strings.ToLower(outFile), ".gz"))
+		outfh, gw, w, err := outStream(outFile, strings.HasSuffix(strings.ToLower(outFile), ".gz"), opt.CompressionLevel)
 		checkError(err)
 		defer func() {
 			outfh.Flush()
diff --git a/unikmer/cmd/inter.go b/unikmer/cmd/inter.go
index 907f1b6..3b99f3b 100644
--- a/unikmer/cmd/inter.go
+++ b/unikmer/cmd/inter.go
@@ -96,7 +96,7 @@ var interCmd = &cobra.Command{
 					if !isStdout(outFile) {
 						outFile += extDataFile
 					}
-					outfh, gw, w, err := outStream(outFile, opt.Compress)
+					outfh, gw, w, err := outStream(outFile, opt.Compress, opt.CompressionLevel)
 					checkError(err)
 					defer func() {
 						outfh.Flush()
@@ -260,7 +260,7 @@ var interCmd = &cobra.Command{
 		if !isStdout(outFile) {
 			outFile += extDataFile
 		}
-		outfh, gw, w, err := outStream(outFile, opt.Compress)
+		outfh, gw, w, err := outStream(outFile, opt.Compress, opt.CompressionLevel)
 		checkError(err)
 		defer func() {
 			outfh.Flush()
diff --git a/unikmer/cmd/num.go b/unikmer/cmd/num.go
index a8a75ed..830a6db 100644
--- a/unikmer/cmd/num.go
+++ b/unikmer/cmd/num.go
@@ -58,7 +58,7 @@ var numCmd = &cobra.Command{
 		outFile := getFlagString(cmd, "out-file")
 		showFile := getFlagBool(cmd, "file-name")
 
-		outfh, gw, w, err := outStream(outFile, strings.HasSuffix(strings.ToLower(outFile), ".gz"))
+		outfh, gw, w, err := outStream(outFile, strings.HasSuffix(strings.ToLower(outFile), ".gz"), opt.CompressionLevel)
 		checkError(err)
 		defer func() {
 			outfh.Flush()
diff --git a/unikmer/cmd/root.go b/unikmer/cmd/root.go
index fac98af..a665d5e 100644
--- a/unikmer/cmd/root.go
+++ b/unikmer/cmd/root.go
@@ -25,6 +25,7 @@ import (
 	"os"
 	"runtime"
 
+	"github.com/klauspost/compress/flate"
 	"github.com/spf13/cobra"
 )
 
@@ -70,6 +71,7 @@ func init() {
 	RootCmd.PersistentFlags().IntP("threads", "j", defaultThreads, "number of CPUs to use. (default value: 1 for single-CPU PC, 2 for others)")
 	RootCmd.PersistentFlags().BoolP("verbose", "", false, "print verbose information")
 	RootCmd.PersistentFlags().BoolP("no-compress", "C", false, "do not compress binary file (not recommended)")
+	RootCmd.PersistentFlags().IntP("compression-level", "L", flate.DefaultCompression, "compression level")
 	RootCmd.PersistentFlags().BoolP("compact", "c", false, "write more compact binary file with little loss of speed")
 	RootCmd.PersistentFlags().StringP("infile-list", "i", "", "file of input files list (one file per line), if given, files from cli arguments are ignored")
 }
diff --git a/unikmer/cmd/sample.go b/unikmer/cmd/sample.go
index 8342029..4573e3d 100644
--- a/unikmer/cmd/sample.go
+++ b/unikmer/cmd/sample.go
@@ -72,7 +72,7 @@ Attentions:
 		if !isStdout(outFile) {
 			outFile += extDataFile
 		}
-		outfh, gw, w, err := outStream(outFile, opt.Compress)
+		outfh, gw, w, err := outStream(outFile, opt.Compress, opt.CompressionLevel)
 		checkError(err)
 		defer func() {
 			outfh.Flush()
diff --git a/unikmer/cmd/sort.go b/unikmer/cmd/sort.go
index c640c0e..1b30ebd 100644
--- a/unikmer/cmd/sort.go
+++ b/unikmer/cmd/sort.go
@@ -63,7 +63,7 @@ var sortCmd = &cobra.Command{
 		if !isStdout(outFile) {
 			outFile += extDataFile
 		}
-		outfh, gw, w, err := outStream(outFile, opt.Compress)
+		outfh, gw, w, err := outStream(outFile, opt.Compress, opt.CompressionLevel)
 		checkError(err)
 		defer func() {
 			outfh.Flush()
diff --git a/unikmer/cmd/stats.go b/unikmer/cmd/stats.go
index e7b1924..c5351e6 100644
--- a/unikmer/cmd/stats.go
+++ b/unikmer/cmd/stats.go
@@ -75,7 +75,7 @@ Tips:
 			checkError(fmt.Errorf("values of -/--symbol-true and -F/--symbol--false should no be the same"))
 		}
 
-		outfh, gw, w, err := outStream(outFile, strings.HasSuffix(strings.ToLower(outFile), ".gz"))
+		outfh, gw, w, err := outStream(outFile, strings.HasSuffix(strings.ToLower(outFile), ".gz"), opt.CompressionLevel)
 		checkError(err)
 		defer func() {
 			outfh.Flush()
diff --git a/unikmer/cmd/subset.go b/unikmer/cmd/subset.go
index 6490e84..c6b998a 100644
--- a/unikmer/cmd/subset.go
+++ b/unikmer/cmd/subset.go
@@ -93,7 +93,7 @@ Attention:
 		if !isStdout(outFile) {
 			outFile += extDataFile
 		}
-		outfh, gw, w, err := outStream(outFile, opt.Compress)
+		outfh, gw, w, err := outStream(outFile, opt.Compress, opt.CompressionLevel)
 		checkError(err)
 		defer func() {
 			outfh.Flush()
diff --git a/unikmer/cmd/union.go b/unikmer/cmd/union.go
index 7051d5b..bdd6829 100644
--- a/unikmer/cmd/union.go
+++ b/unikmer/cmd/union.go
@@ -68,7 +68,7 @@ Attentions:
 		if !isStdout(outFile) {
 			outFile += extDataFile
 		}
-		outfh, gw, w, err := outStream(outFile, opt.Compress)
+		outfh, gw, w, err := outStream(outFile, opt.Compress, opt.CompressionLevel)
 		checkError(err)
 		defer func() {
 			outfh.Flush()
diff --git a/unikmer/cmd/util-io.go b/unikmer/cmd/util-io.go
index 1b92784..85fd71c 100644
--- a/unikmer/cmd/util-io.go
+++ b/unikmer/cmd/util-io.go
@@ -31,7 +31,7 @@ import (
 	gzip "github.com/klauspost/pgzip"
 )
 
-func outStream(file string, gzipped bool) (*bufio.Writer, io.WriteCloser, *os.File, error) {
+func outStream(file string, gzipped bool, level int) (*bufio.Writer, io.WriteCloser, *os.File, error) {
 	var w *os.File
 	if file == "-" {
 		w = os.Stdout
@@ -52,7 +52,11 @@ func outStream(file string, gzipped bool) (*bufio.Writer, io.WriteCloser, *os.Fi
 	}
 
 	if gzipped {
-		gw := gzip.NewWriter(w)
+		// gw := gzip.NewWriter(w)
+		gw, err := gzip.NewWriterLevel(w, level)
+		if err != nil {
+			return nil, nil, nil, fmt.Errorf("fail to write %s: %s", file, err)
+		}
 		return bufio.NewWriterSize(gw, os.Getpagesize()), gw, w, nil
 	}
 	return bufio.NewWriterSize(w, os.Getpagesize()), nil, w, nil
diff --git a/unikmer/cmd/util.go b/unikmer/cmd/util.go
index 8973c2e..6a37411 100644
--- a/unikmer/cmd/util.go
+++ b/unikmer/cmd/util.go
@@ -21,6 +21,7 @@
 package cmd
 
 import (
+	"compress/flate"
 	"fmt"
 	"io"
 	"sort"
@@ -41,19 +42,25 @@ const (
 
 // Options contains the global flags
 type Options struct {
-	NumCPUs  int
-	Verbose  bool
-	Compress bool
-	Compact  bool
+	NumCPUs          int
+	Verbose          bool
+	Compress         bool
+	Compact          bool
+	CompressionLevel int
 }
 
 func getOptions(cmd *cobra.Command) *Options {
+	level := getFlagInt(cmd, "compression-level")
+	if level < flate.HuffmanOnly || level > flate.BestCompression {
+		checkError(fmt.Errorf("gzip: invalid compression level: %d", level))
+	}
 	return &Options{
 		NumCPUs: getFlagPositiveInt(cmd, "threads"),
 		// NumCPUs: 1,
-		Verbose:  getFlagBool(cmd, "verbose"),
-		Compress: !getFlagBool(cmd, "no-compress"),
-		Compact:  getFlagBool(cmd, "compact"),
+		Verbose:          getFlagBool(cmd, "verbose"),
+		Compress:         !getFlagBool(cmd, "no-compress"),
+		Compact:          getFlagBool(cmd, "compact"),
+		CompressionLevel: level,
 	}
 }
 
@@ -168,7 +175,7 @@ func sortUnikFile(opt Options, unique bool, file string, outFile string) (*unikm
 	if !isStdout(outFile) {
 		outFile += extDataFile
 	}
-	outfh, gw, w, err := outStream(outFile, opt.Compress)
+	outfh, gw, w, err := outStream(outFile, opt.Compress, opt.CompressionLevel)
 	if err != nil {
 		return nil, 0, err
 	}
diff --git a/unikmer/cmd/view.go b/unikmer/cmd/view.go
index 2069203..1406f63 100644
--- a/unikmer/cmd/view.go
+++ b/unikmer/cmd/view.go
@@ -61,7 +61,7 @@ var viewCmd = &cobra.Command{
 		outFasta := getFlagBool(cmd, "fasta")
 		outFastq := getFlagBool(cmd, "fastq")
 
-		outfh, gw, w, err := outStream(outFile, strings.HasSuffix(strings.ToLower(outFile), ".gz"))
+		outfh, gw, w, err := outStream(outFile, strings.HasSuffix(strings.ToLower(outFile), ".gz"), opt.CompressionLevel)
 		checkError(err)
 		defer func() {
 			outfh.Flush()