A cooling fan (10) includes a bottom plate (12) a top cover (14) covering the bottom plate, a sidewall (13) disposed between the bottom plate and the top cover, and an impeller (16) enclosed in a space formed between the bottom plate, the top cover and the sidewall. The impeller includes a hub (162), and a plurality of blades (164) radially and outwardly extending from the hub. Each of the blades includes a windward surface (164a) and a leeward surface (164b) opposite to the windward surface. The leeward surface of each blade defines three indents (171, 172, 173) therein. Two protrusions (174) are formed between adjacent indents. The uneven leeward surface is provided for lowering the noise level generated by the cooling fan during an operation thereof.