Abstract: This paper analyzes the issue of calculating the threshold for energy detection when an estimated noise power is used. The corresponding closed-form detection performances of energy detection are derived in the terms of Q function. To achieve the expected detection performance, the threshold cannot be derived by replacing the exact noise power with the estimated ones and must be modified. Moreover, the closed-form expressions of modified thresholds are given in this paper, which simplify the analysis of the detection performances and the calculation of the threshold. Simulation results show that when the sample number is 20, compared to the original threshold, the false alarm probability based on the modified threshold decreases 15%. The throughput of Cognitive Radio (CR) networks can be effectively increased.