diff --git a/packages/runtime-dom/__tests__/customElement.spec.ts b/packages/runtime-dom/__tests__/customElement.spec.ts
index 777f9677d04..4fd852308c3 100644
--- a/packages/runtime-dom/__tests__/customElement.spec.ts
+++ b/packages/runtime-dom/__tests__/customElement.spec.ts
@@ -1,6 +1,7 @@
 import {
   defineAsyncComponent,
   defineCustomElement,
+  defineComponent,
   h,
   inject,
   nextTick,
@@ -339,6 +340,51 @@ describe('defineCustomElement', () => {
       const style = el.shadowRoot?.querySelector('style')!
       expect(style.textContent).toBe(`div { color: red; }`)
     })
+
+    test('should attach styles of children components to shadow dom', () => {
+      const Bar = defineComponent({
+        styles: [`.green-color { color: green; }`],
+        render() {
+          return h(
+            'h1',
+            {
+              attrs: { class: 'green-color' }
+            },
+            'hello'
+          )
+        }
+      })
+      const Foo = defineComponent({
+        components: { Bar },
+        styles: [`.blue-back { color: blue; }`],
+        render() {
+          return h(
+            'span',
+            {
+              attrs: { class: 'blue-back' }
+            },
+            '<bar></bar>'
+          )
+        }
+      })
+
+      const FooBar = defineCustomElement({
+        components: { Foo },
+        styles: [`div { color: red; }`],
+        render() {
+          return h('div', '<foo/>')
+        }
+      })
+      customElements.define('my-el-with-nested-styles', FooBar)
+      container.innerHTML = `<my-el-with-nested-styles></my-el-with-nested-styles>`
+      const el = container.childNodes[0] as VueElement
+      const style = el.shadowRoot?.querySelectorAll('style')!
+
+      expect(style.length).toBe(3)
+      expect(style[0].textContent).toBe(`.green-color { color: green; }`)
+      expect(style[1].textContent).toBe(`.blue-back { color: blue; }`)
+      expect(style[2].textContent).toBe(`div { color: red; }`)
+    })
   })
 
   describe('async', () => {
diff --git a/packages/runtime-dom/src/apiCustomElement.ts b/packages/runtime-dom/src/apiCustomElement.ts
index 059fcac16f1..89e267fd139 100644
--- a/packages/runtime-dom/src/apiCustomElement.ts
+++ b/packages/runtime-dom/src/apiCustomElement.ts
@@ -19,7 +19,8 @@ import {
   nextTick,
   warn,
   ConcreteComponent,
-  ComponentOptions
+  ComponentOptions,
+  Component
 } from '@vue/runtime-core'
 import { camelize, extend, hyphenate, isArray, toNumber } from '@vue/shared'
 import { hydrate, render } from '.'
@@ -215,6 +216,7 @@ export class VueElement extends BaseClass {
     }).observe(this, { attributes: true })
 
     const resolve = (def: InnerComponentDef) => {
+
       const { props, styles } = def
       const hasOptions = !isArray(props)
       const rawKeys = props ? (hasOptions ? Object.keys(props) : props) : []
@@ -250,12 +252,11 @@ export class VueElement extends BaseClass {
           }
         })
       }
-
-      // apply CSS
-      this._applyStyles(styles)
+      this._applyStyles(this._getChildrenComponentsStyles(def))
 
       // initial render
       this._update()
+
     }
 
     const asyncDef = (this._def as ComponentOptions).__asyncLoader
@@ -375,4 +376,42 @@ export class VueElement extends BaseClass {
       })
     }
   }
+
+  private _getChildrenComponentsStyles(
+    component: Component & {
+      components?: Record<string, Component>
+      styles?: string[]
+    }
+  ): string[] {
+    let componentStyles: string[] = []
+
+    if (component.components) {
+      componentStyles = Object.values(component.components).reduce(
+        (
+          aggregatedStyles: string[],
+          nestedComponent: Component & {
+            components?: Record<string, Component>
+            styles?: string[]
+          }
+        ) => {
+          if (nestedComponent?.components) {
+            aggregatedStyles = [
+              ...aggregatedStyles,
+              ...this._getChildrenComponentsStyles(nestedComponent)
+            ]
+          }
+          return nestedComponent.styles
+            ? [...aggregatedStyles, ...nestedComponent.styles]
+            : aggregatedStyles
+        },
+        [] as string[]
+      )
+    }
+
+    if (component.styles) {
+      componentStyles.push(...component.styles)
+    }
+
+    return [...new Set(componentStyles)]
+  }
 }