This paper introduces a novel control strategy "PWAM" method for HEV/EV motor drive system. This modulation method is quite different from other PWM methods that have been well researched or commonly used for the inverter in HEV/EV system. By using this method, only one phase leg of the inverter is doing switching action for every PWM-carrier period. Therefore, the inverter total switching time is reduced to one third comparing with conventional SPWM method and the switching loss can be reduced by at least 50% or even more depending on the load power factor value. In addition, the inverter dc-link requires much smaller capacitor when PWAM method is applied, which makes the system smaller size and lighter weight. Theoretical analysis, simulation and experimental results are provided to verify the inverter efficiency improvement.