A water-cooling device includes a main body having a pump chamber and a heat exchange chamber, which are partitioned from each other by a water room partitioning board. The water room partitioning board has a communication section for communicating the pump chamber with the heat exchange chamber. A pump unit is disposed in the pump chamber. A heat transfer unit is disposed in the heat exchange chamber. A cooling fluid is filled up in the pump chamber and the heat exchange chamber. The water-cooling device has greatly enhanced heat dissipation performance. Moreover, the water-cooling device is free from the problem of overheating of the stator assembly in operation.