Obtaining Energy Efficiency (EE) is very difficult for wireless communication systems especially for cognitive radio network (CRN). The investigation here is with the mean EE maximization problem for secondary user (SU) with both primary user (PU) and secondary users. The solution to the problem starts by considering both the peak and mean transmit power constraints for the SUs and outage probability constraint for the PUs. The problem here is nontrivial. Dinkelbach's algorithm is used to achieve the maximized EE with low number of iterations. Further, it attains the optimal solution via an exhaustive search. Thus the problem is solved at a low complexity. © 2016 IEEE.