Abstract: A novel opportunistic spectrum access strategy based on energy efficiency was proposed to solve the energy constrain issue of a secondary user in cognitive radio ad hoc network, in which a secondary user sensed the channels licensed to some primary users sequentially before it decided to transmit. We developed an analytical framework to maximize energy efficiency based on partially observable Markov decision process (POMDP) through jointly considering the time-variance characteristic of wireless channels and the accuracy of spectrum detection. The framework could lead secondary user to select the channel with the best energy efficiency and adaptively control the transmission power according to channel belief state, channel gain and detection probability while meeting the secondary user’s service requirement. Simulation results indicate that the proposed strategy can improve the energy efficiency of the secondary user’s transmission and achieve the tradeoff between the transmission performance and energy overhead by efficiently regulating the transmission power during its battery life.