diff --git a/expression/builtin_string.go b/expression/builtin_string.go
index 0e5af6c6bb..304939829a 100644
--- a/expression/builtin_string.go
+++ b/expression/builtin_string.go
@@ -1479,15 +1479,8 @@ func (b *builtinLocate2ArgsUTF8Sig) evalInt(row chunk.Row) (int64, bool, error)
 	if int64(len([]rune(subStr))) == 0 {
 		return 1, false, nil
 	}
-	if collate.IsCICollation(b.collation) {
-		str = strings.ToLower(str)
-		subStr = strings.ToLower(subStr)
-	}
-	ret, idx := 0, strings.Index(str, subStr)
-	if idx != -1 {
-		ret = utf8.RuneCountInString(str[:idx]) + 1
-	}
-	return int64(ret), false, nil
+
+	return locateStringWithCollation(str, subStr, b.collation), false, nil
 }
 
 type builtinLocate3ArgsSig struct {
@@ -1569,9 +1562,10 @@ func (b *builtinLocate3ArgsUTF8Sig) evalInt(row chunk.Row) (int64, bool, error)
 		return pos + 1, false, nil
 	}
 	slice := string([]rune(str)[pos:])
-	idx := strings.Index(slice, subStr)
-	if idx != -1 {
-		return pos + int64(utf8.RuneCountInString(slice[:idx])) + 1, false, nil
+
+	idx := locateStringWithCollation(slice, subStr, b.collation)
+	if idx != 0 {
+		return pos + idx, false, nil
 	}
 	return 0, false, nil
 }
diff --git a/expression/integration_test.go b/expression/integration_test.go
index d03e57e2d7..6ebf5e35d1 100644
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -7112,6 +7112,14 @@ func (s *testIntegrationSerialSuite) TestCollateStringFunction(c *C) {
 	tk.MustQuery("select LOCATE(p4,n3) from t1;").Check(testkit.Rows("0", "0", "0"))
 	tk.MustQuery("select LOCATE(p4,n4) from t1;").Check(testkit.Rows("0", "1", "1"))
 
+	tk.MustQuery("select locate('S', 's' collate utf8mb4_general_ci);").Check(testkit.Rows("1"))
+	tk.MustQuery("select locate('S', 'a' collate utf8mb4_general_ci);").Check(testkit.Rows("0"))
+	// MySQL return 0 here, I believe it is a bug in MySQL since 'ß' == 's' under utf8mb4_general_ci collation.
+	tk.MustQuery("select locate('ß', 's' collate utf8mb4_general_ci);").Check(testkit.Rows("1"))
+	tk.MustQuery("select locate('S', 's' collate utf8mb4_unicode_ci);").Check(testkit.Rows("1"))
+	tk.MustQuery("select locate('S', 'a' collate utf8mb4_unicode_ci);").Check(testkit.Rows("0"))
+	tk.MustQuery("select locate('ß', 'ss' collate utf8mb4_unicode_ci);").Check(testkit.Rows("1"))
+
 	tk.MustExec("truncate table t1;")
 	tk.MustExec("insert into t1 (a) values (1);")
 	tk.MustExec("insert into t1 (a,p1,p2,p3,p4,n1,n2,n3,n4) values (2,'0aA1!测试テストמבחן  ','0aA1!测试テストמבחן       ','0aA1!测试テストמבחן  ','0aA1!测试テストמבחן  ','0Aa1!测试テストמבחן','0Aa1!测试テストמבחן','0Aa1!测试テストמבחן','0Aa1!测试テストמבחן');")
@@ -7667,6 +7675,14 @@ func (s *testIntegrationSuite) TestIssue17287(c *C) {
 	tk.MustQuery("execute stmt7 using @val2;").Check(testkit.Rows("1589873946"))
 }
 
+func (s *testIntegrationSuite) TestIssue26989(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("set names utf8mb4 collate utf8mb4_general_ci;")
+	tk.MustQuery("select position('a' in 'AA');").Check(testkit.Rows("0"))
+	tk.MustQuery("select locate('a', 'AA');").Check(testkit.Rows("0"))
+	tk.MustQuery("select locate('a', 'a');").Check(testkit.Rows("1"))
+}
+
 func (s *testIntegrationSuite) TestIssue17898(c *C) {
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("use test")
diff --git a/expression/util.go b/expression/util.go
index 56d5264956..9cb9c0cbe1 100644
--- a/expression/util.go
+++ b/expression/util.go
@@ -14,12 +14,14 @@
 package expression
 
 import (
+	"bytes"
 	"context"
 	"math"
 	"strconv"
 	"strings"
 	"time"
 	"unicode"
+	"unicode/utf8"
 
 	"github.com/pingcap/errors"
 	"github.com/pingcap/failpoint"
@@ -359,6 +361,29 @@ func SubstituteCorCol2Constant(expr Expression) (Expression, error) {
 	return expr, nil
 }
 
+func locateStringWithCollation(str, substr, coll string) int64 {
+	collator := collate.GetCollator(coll)
+	strKey := collator.Key(str)
+	subStrKey := collator.Key(substr)
+
+	index := bytes.Index(strKey, subStrKey)
+	if index == -1 || index == 0 {
+		return int64(index + 1)
+	}
+
+	// todo: we can use binary search to make it faster.
+	count := int64(0)
+	for {
+		r, size := utf8.DecodeRuneInString(str)
+		count += 1
+		index -= len(collator.Key(string(r)))
+		if index == 0 {
+			return count + 1
+		}
+		str = str[size:]
+	}
+}
+
 // timeZone2Duration converts timezone whose format should satisfy the regular condition
 // `(^(+|-)(0?[0-9]|1[0-2]):[0-5]?\d$)|(^+13:00$)` to time.Duration.
 func timeZone2Duration(tz string) time.Duration {