A rear-wheels transmission and suspension system, adapted for modifying a two wheeled motorcycle into a motorcycle containing two rear wheels, includes a suspension device, a rear axle and a transmission device. The suspension device includes a supporting frame longitudinally extending along the central axis of a construction frame and pivotally connected with a seat portion of the construction frame of the motorcycle for supporting the rear axle at the rear end the supporting frame while defining a transmission cavity after a rear end of the supporting frame and at a central position behind an output of said engine along the central axis of the construction frame. Therefore the transmission device is able to be supported by the rear axle in the transmission cavity to transmit a rotation output from the engine to drive the rear axle to rotate for driving the rear wheels to rotate.