Quality of service (QoS) is so significant that it cannot be neglected in the recent evolution of wireless mobile networks, which are characterized by great heterogeneity. Handoff delay is one of the problematic issues that users are facing in the heterogeneous networks. In the wireless networks, many mobile users still cannot obtain the network resources they need when applying real-time services while handoff occurs. Consequently, providing a seamless handoff for QoS connections in heterogeneous wireless local area network (WLAN) and worldwide interoperability for microwave access (WiMAX) is crucial. In this paper, a resource ReSerVation Protocol (RSVP) extension scheme for seamless handoff in heterogeneous WLAN/WiMAX networks is proposed. The proposed scheme is based on QoS aware mobility architecture to guarantee a certain degree of QoS. The network selection is initiated by the mobility of the mobile station (MS) and pre-handoff is performed, basing on received signal strength (RSS). Simulation results show that the proposed scheme outperforms the previous schemes.