A cooling fan includes a housing, a cover arranged on the housing, and an impeller received in a space formed between the housing and the cover. The impeller includes a hub and a plurality of blades extending radially and outwardly from the hub. Each of the blades includes a windward surface and a leeward surface opposite to the windward surface. A porous layer is disposed on the leeward surface of each of the blades, adjacent to a free end thereof. The porous layer has one side surface attached to the leeward surface and an opposite side surface facing the windward surface of an adjacent blade.