The pulse width modulation (PWM) inverter is widely used for the control of motors. We showed that the iron loss under PWM inverter excitation is affected by the impedance of the excitation circuit. The dc voltage of PWM inverter, which is widely used, is constant and the control of motor is carried out by changing the modulation index. Therefore, the evaluation of iron loss under PWM excitation with constant dc voltage is necessary in order to investigate the behavior of iron loss in an actual motor. In this paper, we measured iron losses of non-oriented electrical steel sheets under the shingle-phase full-bridge PWM inverter excitation and examined the influence of modulation index m, carrier frequency fc and dc voltage Vdc on the iron losses. It is shown that the iron loss under PWM inverter excitation can be reduced when dc voltage Vdc is reduced and modulation index m is increased.