Currently, the power density of interior permanent magnet synchronous motors (IPMSMs) for automobiles is improved by downsizing and high-speed rotation. It is necessary to examine the rotor structure and motor size suitable for higher-speed rotation in the future. This study examined the rotor structure of a single-layer V- shaped arrangement of permanent magnets (1V) and a two- layer arrangement (2D). We investigated the influence of iron and copper losses on downsized motors having these rotor structures. As a result, we found that a 2D model with stator diameter reduced by 80 % had the highest efficiency. Furthermore, in consideration of the mechanical strength of the downsized model, the design’s influence on driving characteristics was investigated. In addition, we examined the driving characteristics of a model with stronger magnets installed in this model. The 2D model was proved to be suitable for downsizing.