Cognitive radio (CR) is an emerging technique to improve the utilization of radio frequency spectrum in wireless communication networks. In this paper, we consider the problem of throughput maximization of spectrum allocation in cognitive radio networks. Furthermore, with increasing power level of secondary transmitters, primary receivers are affected by interference and go under outage. Therefore, by taking this into account, we propose a net throughput definition to show throughput change versus SINR threshold. We conduct experiments and evaluate corresponding results.