We investigated the torque characteristics and the motor efficiency of interior permanent magnet synchronous motors (IPMSM), in which a rotor core consists of high-strength nonoriented electrical steel, as-cold-rolled nonoriented electrical steel, and conventional nonoriented electrical steel with various bridge widths. Furthermore, we analyzed deformation behavior of the rotor core during operation by finite element method. The motor of the high-strength nonoriented electrical steel generated greater magnet torque than that of the conventional nonoriented electrical steel. Although the high-strength nonoriented electrical steel had much higher iron loss than the conventional nonoriented electrical steel, the motor efficiency decreased only slightly. A reduction in the bridge width of the rotor core significantly enhanced the torque characteristics and the motor efficiency of IPMSM. The high-strength nonoriented electrical steel effectively suppressed the deformation of the rotor core with its excellent mechanical properties, and the suppression enabled us to reduce the bridge width sufficiently. Accordingly, a magnetically optimum rotor core design will be realized with the high-strength nonoriented electrical steel, and the performance of IPMSM will be significantly enhanced, particularly in traction motors for electric vehicles and hybrid electric vehicles and compressor motors for energy-efficient air conditioners.