This paper proposes a novel time--threshold-based preemption scheme (TTPS) for vertical handoff in a cellular/WLAN interworking networks. in order to improve the QoS of cellular ongoing voice call while guaranteeing system performance and utilization of the bandwidth further, the proposed scheme defines prioriies for cellular ongoing voice calls within the dual cellullar WLAN/ coverage area by using a time-threshold value and modifies channel preemption scheme eliminates DVH of cellular ongoing voice calls which have elapsed real time smaller than the minimum conversation duration accepted moderately by mobile voice user while keeping high bandwidth utilizations and lower call blocking and dropping probabilies when carried traffic loads is relatively lighter compared with PMV scheme. the proposed scheme has much signifinance in providing good QoS guarantess in cellular/WLAN interworking network.