The paper mainly deals with a vertical handover decision algorithm based on the fuzzy control theory. The algorithm takes into consider the factors of Power Level, Cost and Bandwidth. After establishing the membership functions, membership degrees of corresponding factors can be determined, which are processed by the Weight Vector. Finally, the Fuzzy Vertical Handoff Decision Vector is derived and vertical handover decision can be made. It is shown through simulation that the algorithm realizes the optimized vertical handover by evaluating and analyzing various input parameters.