This paper describes the optimal design of interior permanent magnet synchronous motors for which two objective functions regarding motor efficiency and weight are used. Multiobjective optimization technique is applied to finding the optimal solution in this case. An optimal design method that determines both the noninferior set and the best compromise solution employing a modified genetic algorithm is proposed. In order to predict the motor performance more accurately, a core loss formula is derived considering the flux variation due to the armature reaction MMF as well as that due to the magnet.