Lightning surge characteristics of a transmission line comprising a tower, a shield wire and phase conductors are studied with the help of NEC-2. A tower struck by a lightning stroke behaves as an antenna until a traveling wave makes several roundtrips in the tower. During this interval, the tower footing impedance appears to be higher than the footing resistance, and the coupling coefficient between a shield wire and a phase conductor is much lower than that in the TEM mode. These affect the estimation of insulator voltages of transmission towers subject to lightning currents having short rise times. On the basis of these findings, the parameters in the multistory tower model, which has been used widely in EMTP multi-conductor analyzes, are newly proposed.