diff --git a/common/changes/@uifabric/merge-styles/mariust-backgroundSize_2018-05-22-07-00.json b/common/changes/@uifabric/merge-styles/mariust-backgroundSize_2018-05-22-07-00.json
new file mode 100644
index 00000000000000..f436aaa14c232c
--- /dev/null
+++ b/common/changes/@uifabric/merge-styles/mariust-backgroundSize_2018-05-22-07-00.json
@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "packageName": "@uifabric/merge-styles",
+      "comment": "Added missing merge-styles background-size typing",
+      "type": "patch"
+    }
+  ],
+  "packageName": "@uifabric/merge-styles",
+  "email": "mariust@microsoft.com"
+}
\ No newline at end of file
diff --git a/packages/merge-styles/src/IRawStyleBase.ts b/packages/merge-styles/src/IRawStyleBase.ts
index c0a2414ee3dcd8..3d8995dd2ba9ee 100644
--- a/packages/merge-styles/src/IRawStyleBase.ts
+++ b/packages/merge-styles/src/IRawStyleBase.ts
@@ -325,6 +325,11 @@ export interface IRawStyleBase extends IRawFontStyle {
    */
   backgroundRepeat?: ICSSRule | string;
 
+  /**
+   * Sets the size of background images
+   */
+  backgroundSize?: ICSSRule | string;
+
   /**
    * Shorthand property that defines the different properties of all four sides of an
    * element's border in a single declaration. It can be used to set border-width,