Handoff decision making is one of the most challenging issues in wireless heterogeneous networks designs since there are many parameters which have to be considered when triggering handoff and choosing the best candidate access point. Various parameters such as data rate, monetary cost, remaining battery, mobile speed, etc., must be exploited in handoff process of wireless overlapped networks along with the RSSI information which is already utilized widely in traditional networks. Beside these parameters, an effective handoff decision algorithm must be developed to combine and process them as well. In this study, an adaptive fuzzy logic based handoff decision algorithm is introduced for wireless heterogeneous networks. The parameters data rate, monetary cost, RSSI, and mobile speed are considered as inputs of the proposed fuzzy based system. According to these parameters, an output value, which varies between one and ten, is produced. This output describes the candidacy level of the available access points in the vicinity of smart terminal and is utilized in the access point selection algorithm. This algorithm decides whether a handoff is necessary or not, by using the handoff resolution value. The results show that, compared to the traditional RSSI based algorithms the performance is significantly enhanced for both user and network by the method proposed.