According to the quality of service (QoS) requirements of differentiated service (DiffServ), a cross-layer resource allocation algorithm for multi-user orthogonal frequency division multiplexing (OFDM) systems is presented. The constant rate is maintained by adjusting the power dynamically for the voice traffics with high priority, whereas the fairness amongst the data traffics is guaranteed by weighted fairness queued (WFQ) algorithm. The two above-mentioned strategies are used for video traffics to realize variable data rate with the constraint of the minimum rate. Combing all these methods, both the throughput and the fairness are ensured when there are multiple users in the OFDM system. Simulation results indicate the validity of the proposed algorithm, which can work well even if the SNR is less than 0 dB.