A new modified command voltage of the third harmonics injected PWM inverter is proposed to control the inverter in overmodulation mode. The third harmonics injected PWM has a extended linear region compared with the sine PWM. But it still has a maximum voltage about 90% compared with the six step inverter. By analyzing the relationship between the modulation index and the peak of the fundamental component of the modified reference voltage, we can settle the problem in over modulation mode without iteration. Then we can increase the maximum fundamental component of the third harmonics injected PWM inverter comparative to six-step inverter continuously in over modulation mode. The simulation results of the inverter-induction motor system shows the validity of this method.