In the traditional method of bridge design, traffic loads are considered as static loads which are increased by the application of Impact Factor. In current standards, the impact factor is only a function of span length or first flexural natural frequency of the bridge. In this research, the effects of different parameters like vehicle speed, axle distance, number of axles and span length on dynamic responses of steel railway bridges and impact factor are considered. For this purpose, dynamic responses and impact factors are calculated for four simple span steel bridges with 10, 15, 20 and 25 meter length. The vehicle speed varies from 100 to 400 km/h and axle distances from 13 to 24 meter. The results indicate that the impact factors calculated from dynamic analysis in various conditions are greater than those suggested in standards like AASHTO and AREMA so the quantities of impact factors in standards are not sufficient. The impact factor is increased rapidly as the vehicle speed is increased. The results also indicate that the ratio of axle distance to span length affects the quantity of impact factor. The impact factor differs for ratios smaller or greater than one. The number of axles affects the impact factor only in resonance conditions. In this research some formulas are proposed for impact factor considering different parameters such as span length, vehicle speed and axle distance.