diff --git a/src/components/button/button-group.vue b/src/components/button/button-group.vue
index 55f72ff2..d1f54964 100644
--- a/src/components/button/button-group.vue
+++ b/src/components/button/button-group.vue
@@ -14,6 +14,9 @@
             size: {
                 validator (value) {
                     return oneOf(value, ['small', 'large', 'default']);
+                },
+                default () {
+                    return this.$IVIEW.size === '' ? 'default' : this.$IVIEW.size;
                 }
             },
             shape: {
diff --git a/src/components/button/button.vue b/src/components/button/button.vue
index 55eb78f3..76e0fcef 100644
--- a/src/components/button/button.vue
+++ b/src/components/button/button.vue
@@ -86,11 +86,11 @@
             classes () {
                 return [
                     `${prefixCls}`,
+                    `${prefixCls}-${this.type}`,
                     {
-                        [`${prefixCls}-${this.type}`]: !!this.type,
                         [`${prefixCls}-long`]: this.long,
                         [`${prefixCls}-${this.shape}`]: !!this.shape,
-                        [`${prefixCls}-${this.size}`]: !!this.size,
+                        [`${prefixCls}-${this.size}`]: this.size !== 'default',
                         [`${prefixCls}-loading`]: this.loading != null && this.loading,
                         [`${prefixCls}-icon-only`]: !this.showSlot && (!!this.icon || !!this.customIcon || this.loading),
                         [`${prefixCls}-ghost`]: this.ghost