Abstract: The bicycle transmission device includes a hollow tube extending through the bottom bracket and two caps are connected to two ends of the hollow tube. A motor unit has two extensions and multiple fixing assemblies, wherein the extensions are positioned by the caps. The fixing assemblies fix the motor unit to the underside of the bicycle. A shaft extends through the hollow tube and a first one-way transmission device is connected to one end of the shaft. A chainwheel and a first passive wheel are connected to the other end of the transmission shaft. Two cranks are connected to two ends of the transmission shaft. A motor located in the motor unit and has a second passive wheel. The first and second passive wheels transmit energy via the belt. The present invention reduces the space required for the motor and balances the weight of the bicycle.