A modified Third Harmonic Injection PWM (THI-PWM) modulator that reduces output harmonics has been presented in this paper. The modulator is combined with harmonic injection and phase shifting parts in Simulink design. The harmonic injection ratios have been analysed according to harmonic amplitude ratio of third harmonic to fundamental amplitude. The designed modulator is capable to minimize the injected harmonic ratio in order to increase the modulation index over 1.15 limit of regular THI-PWM control scheme. The proposed VSI in this paper supplies 7-level line voltages and 0.25% THDi ratio at 5 kHz while the modulation index is 1.2.