diff --git a/src/cmd/go/internal/modcmd/vendor.go b/src/cmd/go/internal/modcmd/vendor.go
index 7265e62a2f..44cabd5dea 100644
--- a/src/cmd/go/internal/modcmd/vendor.go
+++ b/src/cmd/go/internal/modcmd/vendor.go
@@ -11,6 +11,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"sort"
 	"strings"
 
 	"cmd/go/internal/base"
@@ -72,6 +73,7 @@ func runVendor(cmd *base.Command, args []string) {
 			if cfg.BuildV {
 				fmt.Fprintf(os.Stderr, "# %s %s%s\n", m.Path, m.Version, repl)
 			}
+			sort.Strings(pkgs)
 			for _, pkg := range pkgs {
 				fmt.Fprintf(&buf, "%s\n", pkg)
 				if cfg.BuildV {
diff --git a/src/cmd/go/testdata/script/mod_vendor.txt b/src/cmd/go/testdata/script/mod_vendor.txt
index 25a77a3670..eae4f2946c 100644
--- a/src/cmd/go/testdata/script/mod_vendor.txt
+++ b/src/cmd/go/testdata/script/mod_vendor.txt
@@ -18,6 +18,7 @@ stderr '^y'
 stderr '^# z v1.0.0 => ./z'
 stderr '^z'
 ! stderr '^w'
+grep 'a/foo/bar/b\na/foo/bar/c' vendor/modules.txt # must be sorted
 
 go list -f {{.Dir}} x
 stdout 'src[\\/]x'
@@ -126,6 +127,7 @@ func TestDir(t *testing.T) {
 }
 -- a/foo/bar/c/main.go --
 package c
+import _ "a/foo/bar/b"
 -- a/foo/bar/c/main_test.go --
 package c
 
@@ -190,7 +192,6 @@ import _ "a"
 -- testdata2.go --
 package m
 
-import _ "a/foo/bar/b"
 import _ "a/foo/bar/c"
 -- v1.go --
 package m