diff --git a/mockgen/parse.go b/mockgen/parse.go
index 808fd144..8f31a71d 100644
--- a/mockgen/parse.go
+++ b/mockgen/parse.go
@@ -17,6 +17,7 @@ package main
 // This file contains the model construction by parsing source files.
 
 import (
+	"errors"
 	"flag"
 	"fmt"
 	"go/ast"
@@ -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
 	}
@@ -539,28 +540,29 @@ 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
 // 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(source, srcDir string) (string, error) {
+func parsePackageImport(srcDir string) (string, error) {
 	moduleMode := os.Getenv("GO111MODULE")
 	// trying to find the module
 	if moduleMode != "off" {
 		currentDir := srcDir
-		var modulePath string
 		for {
 			dat, err := ioutil.ReadFile(filepath.Join(currentDir, "go.mod"))
 			if os.IsNotExist(err) {
-				if currentDir == "." || currentDir == "/" {
+				if currentDir == "/" {
 					break
 				}
 				currentDir = filepath.Dir(currentDir)
@@ -568,23 +570,18 @@ func parsePackageImport(source, srcDir string) (string, error) {
 			} else if err != nil {
 				return "", err
 			}
-			modulePath = modfile.ModulePath(dat)
-			break
-		}
-		if modulePath != "" {
+			modulePath := modfile.ModulePath(dat)
 			return filepath.Join(modulePath, strings.TrimPrefix(srcDir, currentDir)), nil
 		}
 	}
-	if moduleMode != "on" {
-		goPath := os.Getenv("GOPATH")
-		if goPath == "" {
-			return "", fmt.Errorf("GOPATH is not set")
-		}
-		sourceRoot := filepath.Join(goPath, "src") + "/"
-		if !strings.HasPrefix(srcDir, sourceRoot) {
-			return "", fmt.Errorf("%s is outside GOPATH %s", srcDir, goPath)
-		}
-		return strings.TrimPrefix(srcDir, sourceRoot), nil
+	// fall back to GOPATH mode
+	goPath := os.Getenv("GOPATH")
+	if goPath == "" {
+		return "", fmt.Errorf("GOPATH is not set")
+	}
+	sourceRoot := filepath.Join(goPath, "src") + "/"
+	if !strings.HasPrefix(srcDir, sourceRoot) {
+		return "", ErrOutsideGoPath
 	}
-	return "", fmt.Errorf("cannot find package path for %s", srcDir)
+	return strings.TrimPrefix(srcDir, sourceRoot), nil
 }
diff --git a/mockgen/parse_test.go b/mockgen/parse_test.go
index dab22fa5..08b51abc 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(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