Abstract: A power driving control system is adapted to be incorporated in an electrically-assisted bicycle which includes an electrical motor mechanically coupled to wheels of the bicycle. The power driving control system includes a first speed detection device for detecting the rotational speed of the pedals actuated by the rider's feet and a second speed detection device to detect the rotational speed of the rear wheel of the bicycle which incorporates a rear derailleur system, and a processor-based controller for receiving the signals from the two speed detection devices and controlling the electrical power supplied from a battery set to the motor in response to the ratio of the two rotational speeds, which may be varied in accordance with the riding condition of the bicycle so that the motor is controlled in a power-efficient way.