The design of optimal sensor operating point in physical layer by considering the impact of the throughput performance in MAC layer is crucial issue. Spectrum sensing action must be performed as quickly as possible to obtain a good throughput performance. However, quick sensing makes the fidelity of sensing outcome becomes low. On the other hand, a long sensing time leads to the low throughput performance. Hence, it is important to find out how secondary users (SUs) in opportunistic spectrum access (OSA) system can improve the throughput performance while maintaining sensing time low. In this paper we study the fundamental design of spectrum sensing and throughput performance for cognitive radio (CR) user with limited sensing capability in OSA system. In the first done, we consider 3 of one slot is allocated for spectrum sensing action and observe the throughput performance by varying the channel measurement sample. The Optimal throughput performance is achieved at certain value of sample number and reduces as increasing it. Subsequently, we observed the throughput performance as a function of spectrum sensing slot allocation as well. Increasing the allocation for spectrum sensing action can degrade the throughput performance. Computer simulations are presented to evaluate the performances. © 2011 IEEE.