An ATP-EMTP simulation model is proposed for calculating lightning transient effects on transmission lines. Both frequency dependent of transmission line and impulse corona effect are considered in this model. The frequency dependent characteristic is analyzed by Marti line model and a user defined corona model is applied to simulate the effect of impulse corona. The expression of geometric capacitance is derived by means of image theory and the dynamic capacitance for line with corona is computed on this base. The simulation results show that the effect of corona plays a more important role than that of frequency dependent characteristic on lightning overvoltage, while it accords more with the facts to take both the effect of frequency dependent characteristic and that of impulse corona. The model presented in this paper can calculate the possible overvoltages on transmission line to provide reliable guidelines for insulating and lightning protection design of transmission line. 