In order to limit the interference caused by cognitive user within the range that primary users can tolerate. The mathematical optimization problem is formulated maximizing cognitive user’s capacity under the interference constraint at the primary receiver．When the signaltonoise ratio of primary user changes according to the communication environment, the cognitive user can adaptively adjust its transmission power according to the interference control factor η. The optimal so1ution to the problem is obtained by utilizing the Lagrange method．A subcarrier and power allocation scheme are proposedamong multiple cognitive users, which is the performance tradeoff between fairness and system capacity. Simulation results show that the proposed scheme can efficiently guarantee the fairness of cognitive users and the capacity performance is close to the optimum.