In this paper, we present the performance evaluation of seven spectrum sensing techniques including four single-stage spectrum sensing and three two-stage spectrum sensing for cognitive radio (CR) systems. We focus on the performance evaluation of these techniques under two types of operative schemes including non-cooperative and cooperative of CR systems when the received signal experiences shadowing. Our system considers under nine values of the shadowing effect. In simulation results, we found that ED-MED gives the best performance of probability of detection than other techniques for both operative schemes. Moreover, we consider three fusion rules such as AND, OR and K-of-N in cooperative scheme of CR systems. The simulation results show that the appropriate fusion rule method under shadowing effect is Kof-N rule.