An interior permanent magnet (IPM) brushless DC motor (BLDCM) for electric vehicles was designed, and the influences of the air gap length, the permanent magnet thickness and the number of stator winding turns on system efficiency and the torque-current ratio were analyzed. Comparative analysis of the performances at different load rates was implemented to consider the complex operating conditions of electric vehicles. An optimized prototype was fabricated and tested. The research results have shown that the reduction of the length of the air gap can improve the motor performance at low speed area on the premise of strong mechanical processing capability, the increase of the amount of permanent magnet can improve the overall performance of the motor, and the reduction of the stator winding turns can improve the motor performance at high speed area but at the same time will cause a drop in the torque-current ratio. The overall efficiency at rated speed is higher, showing that the design of rated operating point is reasonable. The torque-current ratio increases with the increase of load rate, and this is related to the reluctance torque component in the output torque, which is an important distinction between interior and surface permanent magnet motors and is also a noticeable problem in design. The test results have shown that the optimized prototype can meet the need of the driving system both in constant torque region and in constant power region.