In this paper the dynamic effect of concentrated moving load on steel plate girder bridges has been investigated. The vehicle has been modeled as a concentrated load on the bridge. The conventional loadings on bridges are static loadings and the dynamic effect of moving load on analysis is considered as the impact factor. The impact factor is not a function of velocity of vehicle in the design codes, however the present study shows that the velocity of vehicle does affect the impact factor. To consider the influence of effective parameters, it is necessary to know the real interaction of bridge-vehicle. In this study the effect of moving load on the deflection of plate girder bridges has been considered in the dynamic equation of motion. Results of this study show that the velocity of moving load affects the deflection of beams. For small velocities, the dynamic response approaches to static response and in higher velocities there is meaningful differences between dynamic and static responses.