In this report, deformation process of a circular metal pate supported at its periphery under uniform impulsive pressure is investigated by numerical simulation and it is clarified the duration of pressure pulse gives a great effect on the deformation process. When the duration is very short, a plastic bending wave generated wear the supported periphery of the plate moves toward its center and the plastic deformation is performed according to the movement of the bending wave. When the duration is longer than the time till the bending wave arrives at the center, the plate is gradually bulged over the whole area according to the pressure action. Thus the mechanism of the plastic deformation changes depending on the duration of pressure pulse.