diff --git a/go.mod b/go.mod
index edfb6d40..76dfc0f5 100644
--- a/go.mod
+++ b/go.mod
@@ -1,7 +1,8 @@
 module github.com/golang/mock
 
 require (
-	golang.org/x/tools v0.0.0-20190425150028-36563e24a262
+	golang.org/x/mod v0.2.0
+	golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e
 	rsc.io/quote/v3 v3.1.0
 )
 
diff --git a/go.sum b/go.sum
index 0549f947..5d7fe559 100644
--- a/go.sum
+++ b/go.sum
@@ -3,11 +3,15 @@ golang.org/x/net v0.0.0-20190311183353-d8887717615a h1:oWX7TPOiFAMXLq8o0ikBYfCJV
 golang.org/x/net v0.0.0-20190311183353-d8887717615a/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
 golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.0 h1:g61tztE5qeGQ89tm6NTjjM9VPIm088od1l6aSorWRWg=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
-golang.org/x/tools v0.0.0-20190425150028-36563e24a262 h1:qsl9y/CJx34tuA7QCPNp86JNJe4spst6Ff8MjvPUdPg=
-golang.org/x/tools v0.0.0-20190425150028-36563e24a262/go.mod h1:RgjU9mgBXZiqYHBnxXauZ1Gv1EHHAz9KjViQ78xBX0Q=
+golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e h1:aZzprAO9/8oim3qStq3wc1Xuxx4QmAGriC4VU4ojemQ=
+golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
+golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898 h1:/atklqdjdhuosWIl6AIbOeHJjicWYPqR9bpxqxYG2pA=
+golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 rsc.io/quote/v3 v3.1.0 h1:9JKUTTIUgS6kzR9mK1YuGKv6Nl+DijDNIc0ghT58FaY=
 rsc.io/quote/v3 v3.1.0/go.mod h1:yEA65RcK8LyAZtP9Kv3t0HmxON59tX3rD+tICJqUlj0=
 rsc.io/sampler v1.3.0 h1:7uVkIFmeBqHfdjD+gZwtXXI+RODJ2Wc4O7MPEh/QiW4=
diff --git a/mockgen/parse.go b/mockgen/parse.go
index 0ba7f805..8ceed121 100644
--- a/mockgen/parse.go
+++ b/mockgen/parse.go
@@ -26,13 +26,14 @@ import (
 	"go/token"
 	"io/ioutil"
 	"log"
+	"os"
 	"path"
 	"path/filepath"
 	"strconv"
 	"strings"
 
 	"github.com/golang/mock/mockgen/model"
-	"golang.org/x/tools/go/packages"
+	"golang.org/x/mod/modfile"
 )
 
 var (
@@ -49,7 +50,7 @@ func sourceMode(source string) (*model.Package, error) {
 		return nil, fmt.Errorf("failed getting source directory: %v", err)
 	}
 
-	packageImport, err := parsePackageImport(source, srcDir)
+	packageImport, err := parsePackageImport(srcDir)
 	if err != nil {
 		return nil, err
 	}
@@ -604,31 +605,49 @@ func packageNameOfDir(srcDir string) (string, error) {
 		return "", fmt.Errorf("go source file not found %s", srcDir)
 	}
 
-	packageImport, err := parsePackageImport(goFilePath, srcDir)
+	packageImport, err := parsePackageImport(srcDir)
 	if err != nil {
 		return "", err
 	}
 	return packageImport, nil
 }
 
+var errOutsideGoPath = errors.New("Source directory is outside GOPATH")
+
 // parseImportPackage get package import path via source file
-func parsePackageImport(source, srcDir string) (string, error) {
-	cfg := &packages.Config{
-		Mode:  packages.NeedName,
-		Tests: true,
-		Dir:   srcDir,
+// an alternative implementation is to use:
+// cfg := &packages.Config{Mode: packages.NeedName, Tests: true, Dir: srcDir}
+// pkgs, err := packages.Load(cfg, "file="+source)
+// However, it will call "go list" and slow down the performance
+func parsePackageImport(srcDir string) (string, error) {
+	moduleMode := os.Getenv("GO111MODULE")
+	// trying to find the module
+	if moduleMode != "off" {
+		currentDir := srcDir
+		for {
+			dat, err := ioutil.ReadFile(filepath.Join(currentDir, "go.mod"))
+			if os.IsNotExist(err) {
+				if currentDir == filepath.Dir(currentDir) {
+					// at the root
+					break
+				}
+				currentDir = filepath.Dir(currentDir)
+				continue
+			} else if err != nil {
+				return "", err
+			}
+			modulePath := modfile.ModulePath(dat)
+			return filepath.ToSlash(filepath.Join(modulePath, strings.TrimPrefix(srcDir, currentDir))), nil
+		}
 	}
-	pkgs, err := packages.Load(cfg, "file="+source)
-	if err != nil {
-		return "", err
+	// fall back to GOPATH mode
+	goPath := os.Getenv("GOPATH")
+	if goPath == "" {
+		return "", fmt.Errorf("GOPATH is not set")
 	}
-	if packages.PrintErrors(pkgs) > 0 || len(pkgs) == 0 {
-		return "", errors.New("loading package failed")
+	sourceRoot := filepath.Join(goPath, "src") + string(os.PathSeparator)
+	if !strings.HasPrefix(srcDir, sourceRoot) {
+		return "", errOutsideGoPath
 	}
-
-	packageImport := pkgs[0].PkgPath
-
-	// It is illegal to import a _test package.
-	packageImport = strings.TrimSuffix(packageImport, "_test")
-	return packageImport, nil
+	return filepath.ToSlash(strings.TrimPrefix(srcDir, sourceRoot)), nil
 }
diff --git a/mockgen/parse_test.go b/mockgen/parse_test.go
index e555b45e..8a72c06a 100644
--- a/mockgen/parse_test.go
+++ b/mockgen/parse_test.go
@@ -4,6 +4,9 @@ import (
 	"go/ast"
 	"go/parser"
 	"go/token"
+	"io/ioutil"
+	"os"
+	"path/filepath"
 	"testing"
 )
 
@@ -113,3 +116,69 @@ func Benchmark_parseFile(b *testing.B) {
 		sourceMode(source)
 	}
 }
+
+func TestParsePackageImport(t *testing.T) {
+	for _, testCase := range []struct {
+		name    string
+		envs    map[string]string
+		dir     string
+		pkgPath string
+		err     error
+	}{
+		{
+			name:    "go mod default",
+			envs:    map[string]string{"GO111MODULE": ""},
+			dir:     "testdata/gomod/bar",
+			pkgPath: "github.com/golang/foo/bar",
+		},
+		{
+			name:    "go mod off",
+			envs:    map[string]string{"GO111MODULE": "off", "GOPATH": "testdata/gopath"},
+			dir:     "testdata/gopath/src/example.com/foo",
+			pkgPath: "example.com/foo",
+		},
+		{
+			name: "outside GOPATH",
+			envs: map[string]string{"GO111MODULE": "off", "GOPATH": "testdata/gopath"},
+			dir:  "testdata",
+			err:  errOutsideGoPath,
+		},
+	} {
+		t.Run(testCase.name, func(t *testing.T) {
+			for key, value := range testCase.envs {
+				os.Setenv(key, value)
+			}
+			pkgPath, err := parsePackageImport(filepath.Clean(testCase.dir))
+			if err != testCase.err {
+				t.Errorf("expect %v, got %v", testCase.err, err)
+			}
+			if pkgPath != testCase.pkgPath {
+				t.Errorf("expect %s, got %s", testCase.pkgPath, pkgPath)
+			}
+		})
+	}
+}
+
+func TestParsePackageImport_FallbackGoPath(t *testing.T) {
+	goPath, err := ioutil.TempDir("", "gopath")
+	if err != nil {
+		t.Error(err)
+	}
+	defer func() {
+		if err = os.RemoveAll(goPath); err != nil {
+			t.Error(err)
+		}
+	}()
+	srcDir := filepath.Join(goPath, "src/example.com/foo")
+	err = os.MkdirAll(srcDir, 0755)
+	if err != nil {
+		t.Error(err)
+	}
+	os.Setenv("GOPATH", goPath)
+	os.Setenv("GO111MODULE", "on")
+	pkgPath, err := parsePackageImport(srcDir)
+	expected := "example.com/foo"
+	if pkgPath != expected {
+		t.Errorf("expect %s, got %s", expected, pkgPath)
+	}
+}
diff --git a/mockgen/testdata/gomod/bar/bar.go b/mockgen/testdata/gomod/bar/bar.go
new file mode 100644
index 00000000..ddac0faf
--- /dev/null
+++ b/mockgen/testdata/gomod/bar/bar.go
@@ -0,0 +1 @@
+package bar
diff --git a/mockgen/testdata/gomod/go.mod b/mockgen/testdata/gomod/go.mod
new file mode 100644
index 00000000..d80591b4
--- /dev/null
+++ b/mockgen/testdata/gomod/go.mod
@@ -0,0 +1 @@
+module github.com/golang/foo