Recently overlay/underlay framework in Cognitive Radio have been studied and it demonstrated the benefits such as spectrum efficiency and channel capacity maximization. Arising from these work, we suppose the secondary users can operate under overlay mode when the primary user is absent and operate under underlay mode when the primary user is present. In this paper, a hybrid overlay/underlay cognitive radio system is modeled as a M/M/1 queue where the rate of arrival and the service capacity are subject to Poisson alternations. Each packet (as a customer) arriving at the queue makes decision to join or balk the queue. Upon arrival, the individual decision of each packet is optimized based on his observation about the queue length and the state of system. We will present the individual strategy of each customer in detail in this paper.