A driving mechanism for motorized bicycle is provided. The driving mechanism comprises a motor driving transmission device, a front driving sprocket and a pedal driving transmission device. The motor driving transmission device inwardly includes a motor shell, a magnet stator, an electromagnetic wire-coiled rotor and a cylindrical hollow rotatable shaft. The front driving sprocket has a transmission rotatable sleeve disposed therein for insertion of the hollow rotatable shaft there between. The pedal driving transmission device includes a pair of pedal cranks and a pedal driving shaft passing through the transmission rotatable sleeve and hollow rotatable shaft. First and Second single direction irreversible transmission devices are arranged such that either the motor driving transmission device or pedal driving transmission device may drive the transmission rotatable sleeve and front driving sprocket in proper rotational direction.