There are a couple of options here, you could make the sprite in the Flash IDE, using the rectangle tool(color doesn't matter unless you are going to be blending), and load it into the document class. Or, you could make the rectangle with the flash.geom.rectangle class, and add it to a sprite. Instead of Loader.mask, you would want to set it to the Loader variable (I apologize for being unclear..) so, loaderSWF.mask = maskingSprite;