diff --git a/expression/builtin_string.go b/expression/builtin_string.go
index 1daf34280ea0d..7482bf13a573d 100644
--- a/expression/builtin_string.go
+++ b/expression/builtin_string.go
@@ -1695,7 +1695,47 @@ type builtinInstrSig struct {
 
 // See https://dev.mysql.com/doc/refman/5.6/en/string-functions.html#function_instr
 func (b *builtinInstrSig) eval(row []types.Datum) (d types.Datum, err error) {
-	return d, errFunctionNotExists.GenByArgs("instr")
+	args, err := b.evalArgs(row)
+	if err != nil {
+		return d, errors.Trace(err)
+	}
+	// INSTR(str, substr)
+	if args[0].IsNull() || args[1].IsNull() {
+		return d, nil
+	}
+
+	var str, substr string
+	if str, err = args[0].ToString(); err != nil {
+		return d, errors.Trace(err)
+	}
+	if substr, err = args[1].ToString(); err != nil {
+		return d, errors.Trace(err)
+	}
+
+	// INSTR performs case **insensitive** search by default, while at least one argument is binary string
+	// we do case sensitive search.
+	var caseSensitive bool
+	if args[0].Kind() == types.KindBytes || args[1].Kind() == types.KindBytes {
+		caseSensitive = true
+	}
+
+	var pos, idx int
+	if caseSensitive {
+		idx = strings.Index(str, substr)
+	} else {
+		idx = strings.Index(strings.ToLower(str), strings.ToLower(substr))
+	}
+	if idx == -1 {
+		pos = 0
+	} else {
+		if caseSensitive {
+			pos = idx + 1
+		} else {
+			pos = utf8.RuneCountInString(str[:idx]) + 1
+		}
+	}
+	d.SetInt64(int64(pos))
+	return d, nil
 }
 
 type loadFileFunctionClass struct {
diff --git a/expression/builtin_string_test.go b/expression/builtin_string_test.go
index 030c3d82ce0ee..98006f143dd6c 100644
--- a/expression/builtin_string_test.go
+++ b/expression/builtin_string_test.go
@@ -984,6 +984,50 @@ func (s *testEvaluatorSuite) TestLpad(c *C) {
 	}
 }
 
+func (s *testEvaluatorSuite) TestInstr(c *C) {
+	defer testleak.AfterTest(c)()
+	tbl := []struct {
+		Args []interface{}
+		Want interface{}
+	}{
+		{[]interface{}{"foobarbar", "bar"}, 4},
+		{[]interface{}{"xbar", "foobar"}, 0},
+
+		{[]interface{}{123456234, 234}, 2},
+		{[]interface{}{123456, 567}, 0},
+		{[]interface{}{1e10, 1e2}, 1},
+		{[]interface{}{1.234, ".234"}, 2},
+		{[]interface{}{1.234, ""}, 1},
+		{[]interface{}{"", 123}, 0},
+		{[]interface{}{"", ""}, 1},
+
+		{[]interface{}{"中文美好", "美好"}, 3},
+		{[]interface{}{"中文美好", "世界"}, 0},
+		{[]interface{}{"中文abc", "a"}, 3},
+
+		{[]interface{}{"live LONG and prosper", "long"}, 6},
+
+		{[]interface{}{"not BINARY string", "binary"}, 5},
+		{[]interface{}{[]byte("BINARY string"), []byte("binary")}, 0},
+		{[]interface{}{[]byte("BINARY string"), []byte("BINARY")}, 1},
+		{[]interface{}{[]byte("中文abc"), []byte("abc")}, 7},
+
+		{[]interface{}{"foobar", nil}, nil},
+		{[]interface{}{nil, "foobar"}, nil},
+		{[]interface{}{nil, nil}, nil},
+	}
+
+	Dtbl := tblToDtbl(tbl)
+	instr := funcs[ast.Instr]
+	for i, t := range Dtbl {
+		f, err := instr.getFunction(datumsToConstants(t["Args"]), s.ctx)
+		c.Assert(err, IsNil)
+		got, err := f.eval(nil)
+		c.Assert(err, IsNil)
+		c.Assert(got, DeepEquals, t["Want"][0], Commentf("[%d]: args: %v", i, t["Args"]))
+	}
+}
+
 func (s *testEvaluatorSuite) TestMakeSet(c *C) {
 	defer testleak.AfterTest(c)()
 
diff --git a/plan/typeinferer.go b/plan/typeinferer.go
index 8592754140913..968bd4a07ab3d 100644
--- a/plan/typeinferer.go
+++ b/plan/typeinferer.go
@@ -380,8 +380,8 @@ func (v *typeInferrer) handleFuncCallExpr(x *ast.FuncCallExpr) {
 		"date_format", "rpad", "lpad", "char_func", "conv", "make_set", "oct", "uuid":
 		tp = types.NewFieldType(mysql.TypeVarString)
 		chs = v.defaultCharset
-	case "strcmp", "isnull", "bit_length", "char_length", "character_length", "crc32", "timestampdiff", "sign",
-		"is_ipv6", "ord":
+	case "strcmp", "isnull", "bit_length", "char_length", "character_length", "crc32", "timestampdiff",
+		"sign", "is_ipv6", "ord", "instr":
 		tp = types.NewFieldType(mysql.TypeLonglong)
 	case "connection_id":
 		tp = types.NewFieldType(mysql.TypeLonglong)
diff --git a/plan/typeinferer_test.go b/plan/typeinferer_test.go
index aa7c3e677620d..1960694f95d4f 100644
--- a/plan/typeinferer_test.go
+++ b/plan/typeinferer_test.go
@@ -227,6 +227,7 @@ func (ts *testTypeInferrerSuite) TestInferType(c *C) {
 		{"char_length('TiDB')", mysql.TypeLonglong, charset.CharsetBin},
 		{"character_length('TiDB')", mysql.TypeLonglong, charset.CharsetBin},
 		{"crc32('TiDB')", mysql.TypeLonglong, charset.CharsetBin},
+		{"instr('foobarbar', 'bar')", mysql.TypeLonglong, charset.CharsetBin},
 		{"timestampdiff(MINUTE,'2003-02-01','2003-05-01 12:05:55')", mysql.TypeLonglong, charset.CharsetBin},
 		{"sign(0)", mysql.TypeLonglong, charset.CharsetBin},
 		{"sign(null)", mysql.TypeLonglong, charset.CharsetBin},