A rotation direction control method of a cooling fan is disclosed. The rotation direction control method includes a detection step, a determination step and a driving step. The detection step receives a temperature control signal from a temperature detection unit by a rotation direction control unit when a predetermined dust-expelling time period begins. The determination step determines whether a detected temperature is higher than a predetermined value based on the temperature control signal by the rotation direction control unit. The driving step controls the rotation direction control unit to keep outputting a cooling signal so as to drive a motor of the cooling fan for a cooling operation when the determination of the determination step is positive.