In this paper, we are interested in evaluating the performance of a cognitive radio network. We look for optimizing the decision process of secondary mobiles between sensing or not primary's channels. We consider first the global system and look for the optimal proportion of secondary mobiles that sense the primary's channels. Second, we assume that each secondary mobile decides opportunistically to sense or not. In this case, the secondary mobiles are in competition. After showing the existence and uniqueness of the equilibrium , we evaluate the performance of this equilibrium by looking at the price of the anarchy.