This paper proposes a simple and efficient method for calculating a transmission line transient including its frequency-dependent effect. The frequency-dependent distributed-parameter line is modeled by a combination of a distributed-parameter line with constant loss and velocity and a lumped-parameter circuit representing the frequency dependence of the original line. The lumped parameters are derived from a four-terminal parameter theory. Calculated examples by the proposed method are compared with the accurate results by a frequency-domain transient program FTP, and by the EMTP. The comparison shows the accuracy of the proposed method is satisfactory.