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In order to quantify the impact of channel and/or diversity scheme on the probability of mission detection, a concept of sensing gain was defined recently by us, very similar to diversity gain, which determines the slope of probability of missing detection versus average signal-to-noise ratio (SNR) curve. The sensing gain is an important performance target, and very useful in the performance analysis of spectrum sensing in cognitive radio (CR) networks. In this paper, we analyze spectrum sensing performance in cooperative CR networks with CR nodes and a fusion center, and focus on the problem of determining their sensing gain. It is assumed that all channel links from the primary user to the CR nodes and from the CR nodes to the fusion center are flat Rayleigh fading, and the fusion center utilizes the out of rule (such as the Majority rule) as its fusion rule. It is shown that (1) the sensing gain can be designed to be equal to N-K+1 for the out of rule; and (2) the sensing gain can become N/2 for the Majority rule by setting an appropriate SNR threshold for the fusion center.