A cooling fan includes: a casing having a base wall and a surrounding wall that extends transversely from the base wall, and that has an inner wall surface and a top opening; a stator mounted on the inner wall surface of the surrounding wall; a shaft protruding from the base wall; a fan blade unit including an annular hub that is journalled rotatably to the shaft, a supporting ring that surrounds the hub, and a plurality of blades that extend between and that are connected to the hub and the supporting ring; a rotor mounted on the supporting ring; and a cover in the form of a circuit board mounted on the top end face of the surrounding wall for covering the top opening in the casing and including a substrate formed with a central opening.