A cooling fan includes a fan housing (30) having a central tube (34) extending upwardly therefrom, a bearing (60) being received in the central tube, a stator (20) being mounted around the central tube, and a rotor (10) having a shaft (18) extending through the bearing. At least one notch (180, 184) is defined in an end of the shaft. A balance structure (62) is mounted around the shaft, and has a portion facing and surrounding an upper portion of the at least one notch of the shaft. The balance structure is made of magnetic material, which can attract the shaft downward to thereby counter the upward force generated by the rotor when it is driven to rotate. Accordingly, the noise problem incurred by the floating of the rotating rotor can be resolved by the present invention.