In order to improve accuracy of vertical handoff and to decrease connection dropping in heterogeneous wireless networks, a vertical handoff scheme was presented. This scheme improves the RSS-based handoff trigger mechanism and adopts the fuzzy logic-based normalized quantitative decision algorithm to determine the optimal handoff target network from all available access networks. To verify the feasibility of this approach, a heterogeneous network comprising UMTS and WLAN was used in simulation. Numerical results have demonstrated that the proposed approach outperforms other ones in the number of vertical handoff and handoff dropping rate while choosing effectively the optimum network by making trade-off among the user preference, network condition and system performance.