Nowadays, in addition to some sensitive situations such as military and disaster applications, wireless communication has become a part of the human life. This has led to increase the demand for spectrum, while almost the entire part of the spectrum over which the communication can be practiced more effectively has been assigned to users and spectrum scarcity is becoming a major challenge now. On the other hand, it has been observed that the assigned spectrum is used in an inefficient way, so that a huge part of it remains unused most of the time. These two observations lead to the dynamic spectrum allocation and sharing ideas, and cognitive radio is proposed to bring these ideas to reality. The idea of Cognitive radio was first proposed in 1999. Since then, several definitions are introduced for the term "cognitive radio", all of whom are common in two specifications: the ability of environment cognition and the ability of reconfiguration of operating status. Cognitive radio, acting as a secondary user, tries to find the spectrum holes (channels assigned to primary users being idle in specific times and locations), through spectrum sensing. It is necessary to perform the sensing accurately and fast, in order not to lose opportunities as well as not to cause interference for primary users. After that, the cognitive radio chooses one of the idle channels and sets its communication parameters accordingly. Several schemes have been proposed for spectrum sensing, including cooperative approaches, in which several radios cooperate to sense the spectrum. One method of cooperation is the centralized one with hard decision, in which cooperating nodes provide their local decisions for a fusion center to combine them and make the final decision about the idelity in a specific band. In addition, because of a probable communication rate constraint in the control channel between the fusion center and nodes, a censoring scheme can be used. In this scheme, each node sends its decisions to the center only if it is informative. In this research, after reviewing existing works in cooperative spectrum sensing area, an optimal decision rule in the center and an optimal censoring strategy in nodes based on Neyman- Pearson criterion are proposed. To do so, first an optimal decision strategy in the center is investigated. Then, an optimal censoring strategy in nodes is obtained. Finally, the joint optimization is performed on the above two strategies and an algorithm is proposed. In addition, based on another criterion the problem is resolved and it is shown that the same algorithm can be adopted for this problem. Comparing the proposed method with some other existing methods shows that the proposed method has much better performance. Key Words: cognitive radio, cooperative spectrum sensing, censoring, optimization