In order to check the cooling effect of the motor by using the spiral water jacket, a three-dimensional temperature field of the motor was studied by finite element method for a ultra-high speed PM motor with a rated 10 kW, 90 000 r/min considered stator iron loss, stator copper loss, rotor iron loss and rotor wind friction loss. The results showed that the air in the stator-rotor narrow gap seems to impose a heat sealed on the rotor. Therefore, it would not very effectively to cool the rotor with a significant loss power by the spiral water jacket only. Some additional cooling method, such as forced air cooling, should be applied in order to reduce the rotor temperature further. The results provideed an important reference for the thermal design of high power high speed PM motor.