Abstract: A motorized wheel chair includes two drive wheels rotatably attached to a frame with wheel axles, and a push rim attached to each drive wheel for being operated by the user to rotate the drive wheels, a motor driving device includes a gear secured to the wheel axle of the drive wheel, a motor attached to the drive wheel and engaged with the gear for driving the drive wheel, and an encoder attached to the drive wheel and engaged with the gear for detecting the rotating of the drive wheel, and a control device includes a processor device coupled to the motor and the encoder for receiving a signal from the encoder and for controlling the motor to operate the drive wheel.