An impact power tool includes a motor, a battery, a hammer mechanism and a control apparatus. The motor runs in the same speed no matter what torque level is selected. The motor runs for a total running time which is identical to an impacting time plus a compensative time. The impacting time is decided according to the selected torque level, and the compensative time is decided according to the power of the battery. An impacting time table and a compensative time table are pre-established to decide the impacting time value and the compensative time value. Therefore, the impact power tool always provides the theory torque output of the selected torque even if the battery is low.