The effective capacity region of the two-user opportunistic spectrum access (OSA) system in Rayleigh fading environment is derived in this paper.Although OSA is a contemporary research topic,little attention has been given to the capacity analysis under quality-of-service (QoS) constraints.Hence,we use the effective capacity,which is a powerful tool to analyze the QoS requirements in wireless communication systems,to study the sustainable packet arrival rate of the secondary users while meeting statistical QoS constraints.We derive the effective capacity region of the two-user OSA system and propose a method to obtain the capacity region boundary.In addition,by combining the finite state channel model and the activity of the primary user,an extended finite state channel model for the two-user OSA system is also proposed. The effective capacity region of the two-user opportunistic spectrum access (OSA) system in Rayleigh fading environment is derived in this paper. Although OSA is a contemporary research topic, little attention has been to to capacity analysis under quality-of-service ( QoS) constraints.Hence, we use the effective capacity, which is a powerful tool to analyze the QoS requirements in wireless communication systems, to study the sustainable packet arrival rate of the secondary users while meeting statistical QoS constraints. We derive the effective capacity region of the two-user OSA system and propose a method to obtain the capacity region boundary. In addition, by combining the finite state channel model and the activity of the primary user, an extended finite state channel model for the two-user OSA system is also proposed.