In order to realize high speed machining,the special requirements for the transmission and sturctrue of CNC machine tool have to be satisfied.A high speed spindle unit driven by a builtin motor is developed.An oilwater heat exchange system is used for cooling the spindle motor.The spindle is supported by Si4N3 ceramic ball angular contact bearings.An oilair lubricator is used to lubricate and cool the spindle bearings.Some special structures are taken for balancing the spindle.