Electrical and magnetic characteristics of motorized spindles have a great effect on machining quality of ultra-precision machine tools. Bad characteristics will result in significant heat losses, low efficiency and large vibration. Study on optimizing electrical and magnetic characteristics of an inverter-fed induction motor is presented in this paper. A method of optimization design of an inverter-fed induction motor is given. The method is used to design a motor used for an ultra-precision machine tool motorized spindle. By comparing the optimal design with the traditional one, it shows that the total electrical and magnetic heat losses are obviously decreased and the efficiency can be improved.