In this paper, a new model for pulsewidth modulation (PWM) control systems is proposed and a PWM controller based on the model is designed by using the VSS-type self tuning control method. The duration of pulse signal of the PWM control input in each sampling period is considered as a new control input of the model. The resulted model is a nonlinear system although it can be linearized when the duration is small enough. To compare with, controllers with the minimum variance control method and the self tuning control method are also designed. Simulation results for a DC motor PWM control system show that the designed PWM control systems with the proposed model are with satisfactory control performance.