A power tool includes a housing, a motor, an output shaft, a gear transmission mechanism and a control system. The gear transmission mechanism is connected between the motor and the output shaft to transmit the rotary power of the motor to the output shaft. The power tool includes a signal generator and a control system electrically coupled to the signal generator and operatively engaged with the gear transmission mechanism such that when the signal generator is manually activated, an electric signal is generated and transmitted to the control system to cause the gear transmission mechanism to vary the gear reduction ratio.