In direct communication, in order to deliver synchronization information to mobile terminals outside the coverage area of the base station, some terminals should propagate the synchronization signal through the synchronization channel. In general, a mobile terminal transmits a synchronization signal based on the received synchronization signal. If the received signal strength is below the propagation threshold, the mobile terminal sends a synchronization signal to extend the coverage of the direct communication network. Therefore, the propagation threshold value is closely related to the performance of direct communication networks. If the threshold value is too low, only a minimal number of terminals transmit synchronization information. Thus, the coverage of that direction communication network is limited. On the other hand, if the threshold value is too high, most terminals will transmit synchronization messages, triggering collisions. However, it is difficult to find the optimal propagation threshold value over the entire direct communication network. Therefore, we propose a heuristic method for finding the most efficient threshold value after declaring the decision variables. The proposed method minimizes the probability of collisions between the mobile terminals’ signals on the synchronization channel. By comparing our simulation results with the numerical ones, we show the validity of the proposed synchronization method.