Cognitive radio is an emerging technology, which aims to upgrade the spectrum utilization by allowing the secondary users to operate at the spectrum bands vacated by the primary users. A cognitive radio system model was simulated and the performance of the energy detector was evaluated by using different wireless channel models. These models include Additive White Gaussian Noise (AWGN) model, Rayleigh fading model, and Rician fading model. The simulation results show that by increasing the signal to noise ratio, the detection capability of the energy detector can be improved and the false alarm probability and the missed detection probability can be reduced. Moreover, the line of sight path strength of the Rician fading has a great effect on the energy detector performance.