The maximum torque of the interior permanent magnet motor (IPMSM) is determined by two significant features which are the motor-design parameters and current phase angle. However, simultaneously determining the above features is challenging due to their complicated coupling effect. In this study, the motor-design parameters are obtained by performing the electromagnetic finite element analysis. And then, the current phase angle is expressed by using the calculated motor-design parameters. Thereby, the optimal current phase angle becomes the design-dependent variable in this work. At the end of the optimization, the optimal design and the optimal current reference are simultaneously obtained, considering their coupling effect. The proposed method was applied to two types of motors with ferrite and neodymium magnets. The optimization results were manufactured and validated experimentally. This study is expected to determine the optimal design and current phase angle of the interior permanent magnet synchronous motors in future work.