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In this paper, parameter optimization of multi-layered interior permanent-magnet synchronous motors for electric vehicle propulsion is carried out to improve torque ripple and efficiency at low- and high-speed regions. First, we establish a torque ripple map based on d-q currents. The ripples of the d-q parameters and harmonics of the phase flux linkages are then analyzed to investigate the relationship between the torque ripple and the parameters. Second, we compose and analyze an iron loss map based on d-q currents, and then investigate the parameters that affect the harmonics of the flux density. In particular, because the high-speed region is highly vulnerable to iron loss, the parameters related to this tendency are also analyzed. Based on the analysis results, we develop a parameter optimization strategy that leads to an optimal design. To analyze the electro-magnetic performances of the motors, two- and three-dimensional finite elements analyses are carried out by employing sinusoidal and inverter-controlled currents, and the improvement achieved are demonstrated by the experimental validations on prototypes.