Current bridge design codes specify the Dynamic Load Factor(DLF) as the function of either span length or fundamental frequency of the bridge. However, several measurements show the DLF is dependent on bridge and vehicle characteristics and road conditions. Therefore, the purpose of this study is to quantity the effect of various parameters on DLFs. Parameters include vehicle weight, vehicle speed, vehicle length, bridge stiffness, bridge weight, road roughness coefficient and the bump at bridge entrance. The analysis is performed for 5 axle semi tractor-trailer and simply supported steel girder bridge with span length ranging 12 to 30m.