Abstract: The invention discloses a bicycle drive system (10) and a kit. The drive system comprises a drive unit (15) comprising a housing, a motor unit (20) having a stator (21), a rotor (22) and an output shaft and accommodated in the housing, and a first stage deceleration unit (30) having an input part connected to the output shaft of the motor unit and an output part; a mounting unit (50) configured to secure and hold the drive unit on a bicycle body (70); and a second stage deceleration unit (40) arranged external to the drive unit and detachably mounted on the first stage deceleration unit for connection to a pedal assembly (80). The drive unit, the mounting unit and the second stage deceleration unit are each provided as a modular part for assembling to the bicycle.