Spectrum sensing is the key technology in cognitive radio systems. Generally, matched filter detection, energy detection and feature detection are traditional scheme of spectrum detection for single node detection in cognitive system. Taking into account the error in the decision reporting channel and the hidden terminal problem, cooperative spectrum sensing could solve these problems in cognitive radio networks. In this paper, the performance of cooperative spectrum sensing is discussed in cognitive radio networks over fading channels. And decision fusion rule is discussed to improve the detection performance of cooperative spectrum sensing.