For the line differential protection relays, sampling synchronization is one of the most critical factors to affect the protection performance. Previously, sampling synchronization using voltage phase difference was proposed. In this paper, simulations for more complex, but actually existing, systems such as transmission line consisting of underground cable and over-head line and three-terminal line are carried out. The proposed method is applicable for two-terminal line even if the line consisting of underground cable and over-head line. However, for three (or more) -terminal line, the simple model, which is able to calculate in the relay, may have some error for synchronization and then, this method is only applicable for the limited system.