At present, lots of works focus on spectrum allocation of wireless networks. In this paper, the authors proposed a Cognitive based spectrum access by opportunistically approach of Heterogeneous Wireless networks based on Fuzzy Logic system. The Cognitive Radio is a technology where a network or a wireless system changes its environment parameters to communicate efficiently by avoiding the interference with the users. By applying FLS (Fuzzy Logic System), the available spectrum utilization is effectively utilized with the help of the three antecedents namely Spectrum utilization efficiency, Degree of mobility, Distance from primary user to the secondary users. The proposed work is compared with normal Spectrum Utilization method.