A power assisted bicycle (10) which modifies an existing manually powered bicycle (12) by replacing the left pedal crank arm with a motor-driven sprocket and left pedal assembly (32). An electric motor (34) is attached to the bicycle's frame (20) and has attached a motor drive sprocket (35) and a one-way clutch (36). A motor roller chain ring (38) is connected in concert to the motor drive sprocket (35) and motor driven sprocket (32), such that the electric motor (34), provides a power assist to the manually powered bicycle, with the one-way clutch (36) permitting unassisted pedaling without resistance of the motor when de-energized. An electrical power source (40) interfaces with an electrical control system (42) for supplying electrical energy to operate and control the electric motor.