diff --git a/src/index.js b/src/index.js
index f0f5fc1d..49e5e440 100644
--- a/src/index.js
+++ b/src/index.js
@@ -163,18 +163,18 @@ class Dayjs {
         (isStartOf ? argumentStart : argumentEnd).slice(slice)
       ), this)
     }
-
+    const { $W, $M, $D } = this
     switch (unit) {
       case C.Y:
         return isStartOf ? instanceFactory(1, 0) :
           instanceFactory(31, 11)
       case C.M:
-        return isStartOf ? instanceFactory(1, this.$M) :
-          instanceFactory(0, this.$M + 1)
+        return isStartOf ? instanceFactory(1, $M) :
+          instanceFactory(0, $M + 1)
       case C.W: {
         const weekStart = this.$locale().weekStart || 0
-        return isStartOf ? instanceFactory(this.$D - (this.$W - weekStart), this.$M) :
-          instanceFactory(this.$D + (6 - (this.$W - weekStart)), this.$M)
+        const gap = ($W < weekStart ? $W + 7 : $W) - weekStart
+        return instanceFactory(isStartOf ? $D - gap : $D + (6 - gap), $M)
       }
       case C.D:
       case C.DATE:
diff --git a/src/locale/ar.js b/src/locale/ar.js
index 545db4b5..78534128 100644
--- a/src/locale/ar.js
+++ b/src/locale/ar.js
@@ -4,6 +4,7 @@ const locale = {
   name: 'ar',
   weekdays: 'الأحد_الإثنين_الثلاثاء_الأربعاء_الخميس_الجمعة_السبت'.split('_'),
   months: 'يناير_فبراير_مارس_أبريل_مايو_يونيو_يوليو_أغسطس_سبتمبر_أكتوبر_نوفمبر_ديسمبر'.split('_'),
+  weekStart: 6,
   relativeTime: {
     future: 'بعد %s',
     past: 'منذ %s',
diff --git a/test/manipulate.test.js b/test/manipulate.test.js
index ae0c8fec..c944027f 100644
--- a/test/manipulate.test.js
+++ b/test/manipulate.test.js
@@ -32,10 +32,15 @@ describe('StartOf EndOf', () => {
   })
 
   it('StartOf week with locale', () => {
-    const testArr = ['zh-cn']
-    testArr.forEach((l) => {
-      expect(dayjs().locale(l).startOf('week').date()).toBe(moment().locale(l).startOf('week').date())
-      expect(dayjs().locale(l).endOf('week').date()).toBe(moment().locale(l).endOf('week').date())
+    const testDate = [undefined, '2019-02-10', '2019-02-11', '2019-02-12', '2019-02-13', '2019-02-14', '2019-02-15', '2019-02-16']
+    const testLocale = ['zh-cn', 'ar', 'en']
+    testDate.forEach((d) => {
+      testLocale.forEach((l) => {
+        expect(dayjs(d).locale(l).startOf('week').date())
+          .toBe(moment(d).locale(l).startOf('week').date())
+        expect(dayjs(d).locale(l).endOf('week').date())
+          .toBe(moment(d).locale(l).endOf('week').date())
+      })
     })
   })
 })