Spectrum sensing is a critical issue in cognitive radio networks. Cooperation among the secondary users is utilized to improve the performance of spectrum sensing. In this paper, we quantify the gain of cooperation in spectrum sensing by introducing the concept of diversity order. With different system performance metrics, we introduce different diversity quantities. We analyze the single-user sensing and the multi-user sensing with soft and hard information fusion strategies using the diversity quantities as our figure of merit. In particular, we discuss the selection of threshold in each sensing scheme with respect to the diversity performance and obtain the quantitative relationship among the diversity performance, threshold, and the number of cooperative users. We also observe and quantify the tradeoff between false alarm and missed detection performance in all spectrum sensing schemes.