A bicycle power unit case includes a controller received therein and a power unit is inserted into the case from a top of the case and electrically connected to the controller. The case includes a bottom-bracket hole through which the bottom bracket and crank extend. The case is connected to a low and central portion of the bicycle. The down tube, the seat tube and the chain stays are respectively inserted and fixed to the case. The power unit case does not affect the center of weight of the bicycle and the power unit can be easily replaced from a top of the case.