An energy detection based spectrum sensing has been found to be vulnerable to the noise power uncertainty. A cooperative spectrum sensing with an energy detector has appeared as one of the solutions to alleviate this difficulty. However, its performance analysis in a fading environment has not been reported yet in the literature. Motivated by this, this paper presents the performance analysis of the scheme by extending our previous work on evaluating the performance of an energy detector in the presence of noise power uncertainty. The analysis shows that the false alarm probability and detection probability gets higher as the sensing time and/or the number of the secondary users in the OR based cooperative spectrum sensing scheme increase when the noise power uncertainty exists.