It has been an on-going issue to develop a high voltage motor with high capacity and reliability. In this study, we investigated the effective coil insulator materials in terms of thermal conductivity. To quantify the contribution of the coil insulator material, two different motors with and without the cooling structure were numerically studied. Based on the measured thermal conductivity of six different coil insulators, we have achieved the effectiveness of thermal conductivity. Consequently, the high voltage motor can be developed with the proposed effectiveness of thermal conductivity regarding coil insulator materials. Our study of fundamental material characteristics will be beneficial in enhancing thermal management technology of a high voltage motor.