A centrifugal fan includes a fan frame (20) and an impeller (40) received in the fan frame. The impeller includes a hub (41) having a circular top wall (410) and a cylindrical side wall (412) extending downwardly from a rim of the circular top wall, a plurality of blades (42) extending radially from the side wall of the hub and a blade ring (44). The blade ring includes an annular top plate (441), a cylindrical sidewall (442) extending downwardly from an outer periphery of the top plate, and a flange (443) extending outwardly from a bottom of the cylindrical sidewall. The blade ring is arranged on top surfaces of the blades received in an air inlet (24a) defined in a top surface of the fan frame, and the top plate of the blade ring and the top surface of the fan frame are coplanar with each other.