—Mobile WiMAX has become one of the 3rd Generation communication systems and its performance has been widely evaluated. The physical overhead is a critical factor that may affect the overall performance significantly. But almost no attention has been paid to the impact of overhead on system performance yet. In this paper, we first analyze main signaling resources needed in physical layer, which consist of the physical overhead. Then the dynamic overhead model for downlink and uplink are proposed respectively and simplified, while maintaining the simulation accuracy. Average overhead amount is obtained through system level simulation using dynamic overhead calculation. Finally, it is proved that the model is reasonable and the average overhead size may be used in stead of dynamic calculation for the sake of reducing simulation complexity as well as keeping evaluation results precise. Keywords-Mobile WiMAX, dynamic physical overhead, PUSC, throughput, system level simulation