Abstract: A Trajectory-Aware Vertical Handoff (TAVH) protocol which provides seamless handoff between Worldwide Interoperability for Microwave Access (WiMAX) and Third Generation Partnership Project (3GPP) evolved packet networks is proposed. To reach this approach, TAVH calculates the position, velocity, handoff signaling delay and RSS of MT and each and every data is taken in account in handoff decision. Moreover, simple network architecture for WiMAX and 3GPP interworking based on proxy mobile IPv-6 is presented. Then, the operation of TAVH protocol is explained in detail. Simulation is provided by using random motion model which is very realistic in order to show that the proposed TAVH method consistently outperforms the other methods in almost every output parameter of this simulation.