In this paper, we model the hierarchical structures inherent in cognitive radio networks as the priority queueing system in which primary users interact with the highest priority and secondary users belong to the lowest priority class. In a M/G/1 system containing one primary user and multiple secondary users, we obtain analytical forms of delay and throughput for different users with the function of traffic and channel conditions. Based on the analysis, the secondary user is considered to act as a relaying terminal to assist the primary communication by adopting an amplify-and-forward TDMA protocol. Cooperative diversity gains are examined next and the benefits of the secondary: improvement of throughput, is discussed with respect of the primary traffic.