In this paper, the performances of cognitive radio networks with different packet level policies for secondary users are analyzed, and two models for cognitive radio networks are proposed to derive the formulas of packet loss ratio, packet delay, throughput and spectrum utilization for secondary users. Moreover, a performance simulation is carried out to evaluate the accuracy of the proposed theoretical models. Analysis and simulation results indicate that （ 1 ） the proposed models can accurately evaluate the performance of cognitive radio networks ; （2） the packet loss ratio, packet delay and spectrum utilization of secondary users all increase with the arrival rates of services; （3） cognitive radio networks help to improve the spectrum utilization remarkably; and （4） the difference in packet level policy of secondary users affects the spectrum utilization slightly but it exerts significant effects on the throughput.