diff --git a/examples/shader/radialblur.go b/examples/shader/radialblur.go
index d794d9555..e4eb63eaa 100644
--- a/examples/shader/radialblur.go
+++ b/examples/shader/radialblur.go
@@ -27,9 +27,8 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 	samples := [...]float{
 		-22, -14, -8, -4, -2, 2, 4, 8, 14, 22,
 	}
-	// TODO: Add len(samples)
 	sum := clr
-	for i := 0; i < 10; i++ {
+	for i := 0; i < len(samples); i++ {
 		pos := texCoord + dir*samples[i]/imageSrcTextureSize()
 		sum += image2TextureBoundsAt(pos)
 	}
diff --git a/examples/shader/radialblur_go.go b/examples/shader/radialblur_go.go
index a1e35a010..75c75b82b 100644
--- a/examples/shader/radialblur_go.go
+++ b/examples/shader/radialblur_go.go
@@ -2,4 +2,4 @@
 
 package main
 
-var radialblur_go = []byte("// Copyright 2020 The Ebiten Authors\n//\n// Licensed under the Apache License, Version 2.0 (the \"License\");\n// you may not use this file except in compliance with the License.\n// You may obtain a copy of the License at\n//\n//     http://www.apache.org/licenses/LICENSE-2.0\n//\n// Unless required by applicable law or agreed to in writing, software\n// distributed under the License is distributed on an \"AS IS\" BASIS,\n// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.\n// See the License for the specific language governing permissions and\n// limitations under the License.\n\n// +build ignore\n\npackage main\n\nvar Time float\nvar Cursor vec2\nvar ScreenSize vec2\n\nfunc Fragment(position vec4, texCoord vec2, color vec4) vec4 {\n\tdir := normalize(position.xy - Cursor)\n\tclr := image2TextureAt(texCoord)\n\n\tsamples := [...]float{\n\t\t-22, -14, -8, -4, -2, 2, 4, 8, 14, 22,\n\t}\n\t// TODO: Add len(samples)\n\tsum := clr\n\tfor i := 0; i < 10; i++ {\n\t\tpos := texCoord + dir*samples[i]/imageSrcTextureSize()\n\t\tsum += image2TextureBoundsAt(pos)\n\t}\n\tsum /= 10 + 1\n\n\tdist := distance(position.xy, Cursor)\n\tt := clamp(dist/256, 0, 1)\n\treturn mix(clr, sum, t)\n}\n")
+var radialblur_go = []byte("// Copyright 2020 The Ebiten Authors\n//\n// Licensed under the Apache License, Version 2.0 (the \"License\");\n// you may not use this file except in compliance with the License.\n// You may obtain a copy of the License at\n//\n//     http://www.apache.org/licenses/LICENSE-2.0\n//\n// Unless required by applicable law or agreed to in writing, software\n// distributed under the License is distributed on an \"AS IS\" BASIS,\n// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.\n// See the License for the specific language governing permissions and\n// limitations under the License.\n\n// +build ignore\n\npackage main\n\nvar Time float\nvar Cursor vec2\nvar ScreenSize vec2\n\nfunc Fragment(position vec4, texCoord vec2, color vec4) vec4 {\n\tdir := normalize(position.xy - Cursor)\n\tclr := image2TextureAt(texCoord)\n\n\tsamples := [...]float{\n\t\t-22, -14, -8, -4, -2, 2, 4, 8, 14, 22,\n\t}\n\tsum := clr\n\tfor i := 0; i < len(samples); i++ {\n\t\tpos := texCoord + dir*samples[i]/imageSrcTextureSize()\n\t\tsum += image2TextureBoundsAt(pos)\n\t}\n\tsum /= 10 + 1\n\n\tdist := distance(position.xy, Cursor)\n\tt := clamp(dist/256, 0, 1)\n\treturn mix(clr, sum, t)\n}\n")
diff --git a/internal/shader/expr.go b/internal/shader/expr.go
index 704880649..c10c69226 100644
--- a/internal/shader/expr.go
+++ b/internal/shader/expr.go
@@ -220,6 +220,24 @@ func (cs *compileState) parseExpr(block *block, expr ast.Expr) ([]shaderir.Expr,
 		// For built-in functions, we can call this in this position. Return an expression for the function
 		// call.
 		if callee.Type == shaderir.BuiltinFuncExpr {
+			if callee.BuiltinFunc == shaderir.Len {
+				if len(args) != 1 {
+					cs.addError(e.Pos(), fmt.Sprintf("number of len's arguments must be 1 but %d", len(args)))
+					return nil, nil, nil, false
+				}
+				if argts[0].Main != shaderir.Array {
+					cs.addError(e.Pos(), fmt.Sprintf("len takes an array but %s", argts[0].String()))
+					return nil, nil, nil, false
+				}
+				return []shaderir.Expr{
+					{
+						Type:      shaderir.NumberExpr,
+						Const:     gconstant.MakeInt64(int64(argts[0].Length)),
+						ConstType: shaderir.ConstTypeInt,
+					},
+				}, []shaderir.Type{{Main: shaderir.Int}}, stmts, true
+			}
+
 			var t shaderir.Type
 			switch callee.BuiltinFunc {
 			case shaderir.BoolF:
diff --git a/internal/shader/testdata/len.expected.vs b/internal/shader/testdata/len.expected.vs
new file mode 100644
index 000000000..03c8ea0ef
--- /dev/null
+++ b/internal/shader/testdata/len.expected.vs
@@ -0,0 +1,9 @@
+void F0(in vec2 l0, out int l1);
+
+void F0(in vec2 l0, out int l1) {
+	int l2[2];
+	l2[0] = 0;
+	l2[1] = 0;
+	l1 = 2;
+	return;
+}
diff --git a/internal/shader/testdata/len.go b/internal/shader/testdata/len.go
new file mode 100644
index 000000000..0b4d187ad
--- /dev/null
+++ b/internal/shader/testdata/len.go
@@ -0,0 +1,6 @@
+package main
+
+func Foo(foo vec2) int {
+	var a [2]int
+	return len(a)
+}
diff --git a/internal/shaderir/program.go b/internal/shaderir/program.go
index c1fbcaa25..633bfb529 100644
--- a/internal/shaderir/program.go
+++ b/internal/shaderir/program.go
@@ -198,6 +198,7 @@ func OpFromToken(t token.Token) (Op, bool) {
 type BuiltinFunc string
 
 const (
+	Len         BuiltinFunc = "len"
 	BoolF       BuiltinFunc = "bool"
 	IntF        BuiltinFunc = "int"
 	FloatF      BuiltinFunc = "float"
@@ -250,7 +251,8 @@ const (
 
 func ParseBuiltinFunc(str string) (BuiltinFunc, bool) {
 	switch BuiltinFunc(str) {
-	case BoolF,
+	case Len,
+		BoolF,
 		IntF,
 		FloatF,
 		Vec2F,