This paper presents an effective joint spectrum allocation and power control (JSAPC) method for OFDM-based cognitive radio (OFDM-CR). In the proposed approach, the JSAPC problem is modeled as a multi-objective optimization problem, namely, a joint optimization problem of maximizing the system throughput and minimizing its total transmit power, while keeping the constraints on the bit error rate, the total transmit power within a budget, and the interference induced to primary users below a certain threshold. For solving the joint optimization problem, an effective iterative method based on particles swarm optimization algorithm is developed. The proposed method offers a number of advantages. For examples, it can adjust secondary user's throughput and transmit power level according to the demand of system. Furthermore, the solution obtained by proposed algorithm is close to optimal solution. Simulation results are presented to show the effectiveness of the presented method.