diff --git a/changelog.md b/changelog.md
index 54c7bc1d9..13dc05d21 100755
--- a/changelog.md
+++ b/changelog.md
@@ -2,6 +2,7 @@
 
 ### Improvements
 
+- [jss] Simplify cloneStyle function ([#1003](https://github.com/cssinjs/jss/pull/1003))
 - [internal] Improve publish script for creating github release ([#999](https://github.com/cssinjs/jss/pull/999))
 
 ### Bug fixes
diff --git a/packages/jss/.size-snapshot.json b/packages/jss/.size-snapshot.json
index 2df38f0fa..92662d827 100644
--- a/packages/jss/.size-snapshot.json
+++ b/packages/jss/.size-snapshot.json
@@ -1,30 +1,30 @@
 {
   "dist/jss.js": {
-    "bundled": 59740,
-    "minified": 22374,
-    "gzipped": 6667
+    "bundled": 58181,
+    "minified": 21925,
+    "gzipped": 6523
   },
   "dist/jss.min.js": {
-    "bundled": 58601,
-    "minified": 21513,
-    "gzipped": 6230
+    "bundled": 57042,
+    "minified": 21067,
+    "gzipped": 6091
   },
   "dist/jss.cjs.js": {
-    "bundled": 54651,
-    "minified": 24090,
-    "gzipped": 6647
+    "bundled": 53766,
+    "minified": 23846,
+    "gzipped": 6579
   },
   "dist/jss.esm.js": {
-    "bundled": 54135,
-    "minified": 23671,
-    "gzipped": 6560,
+    "bundled": 53270,
+    "minified": 23442,
+    "gzipped": 6498,
     "treeshaked": {
       "rollup": {
-        "code": 19192,
-        "import_statements": 314
+        "code": 18995,
+        "import_statements": 281
       },
       "webpack": {
-        "code": 20665
+        "code": 20418
       }
     }
   }
diff --git a/packages/jss/package.json b/packages/jss/package.json
index 9f67af6f8..71f038447 100644
--- a/packages/jss/package.json
+++ b/packages/jss/package.json
@@ -42,7 +42,6 @@
   "dependencies": {
     "@babel/runtime": "^7.0.0",
     "is-in-browser": "^1.1.3",
-    "symbol-observable": "^1.2.0",
     "tiny-warning": "^1.0.2"
   }
 }
diff --git a/packages/jss/src/utils/cloneStyle.js b/packages/jss/src/utils/cloneStyle.js
index 7b034cb2b..37d672625 100644
--- a/packages/jss/src/utils/cloneStyle.js
+++ b/packages/jss/src/utils/cloneStyle.js
@@ -1,44 +1,15 @@
-import $$observable from 'symbol-observable'
 import type {JssStyle} from '../types'
 
-const {isArray} = Array
-
-// TODO: This should propably not be here, need to find a better place
-const isObservable = value => value && value[$$observable] && value === value[$$observable]()
+const plainObjectConstrurctor = {}.constructor
 
 export default function cloneStyle(style: JssStyle): JssStyle {
-  // Support empty values in case user ends up with them by accident.
-  if (style == null) return style
-
-  // Support string value for SimpleRule.
-  const typeOfStyle = typeof style
-
-  if (typeOfStyle === 'string' || typeOfStyle === 'number' || typeOfStyle === 'function') {
-    return style
-  }
-
-  // It is a CSSTOM value.
-  // TODO will not work if instance comes from a different window.
-  if (global.CSSStyleValue && style instanceof global.CSSStyleValue) {
-    return style
-  }
-
-  // Support array for FontFaceRule.
-  if (isArray(style)) return style.map(cloneStyle)
-
-  // Support Observable styles.  Observables are immutable, so we don't need to
-  // copy them.
-  if (isObservable(style)) return style
+  if (style == null || typeof style !== 'object') return style
+  if (Array.isArray(style)) return style.map(cloneStyle)
+  if (style.constructor !== plainObjectConstrurctor) return style
 
   const newStyle = {}
   for (const name in style) {
-    const value = style[name]
-    if (typeof value === 'object') {
-      newStyle[name] = cloneStyle(value)
-      continue
-    }
-    newStyle[name] = value
+    newStyle[name] = cloneStyle(style[name])
   }
-
   return newStyle
 }