In this paper, we consider the service of voice over internet protocol (VoIP) traffic with the service of non-real time traffic in the mobile WiMAX system. VoIP traffic is delay-sensitive and requires transmission at an almost constant rate. As a consequence, it may require a large amount of resource when users are in poor channel condition, leading to resource outage problem. To alleviate this problem, we consider the allocation of power according to the VoIP traffic load as well as the channel condition. When the VoIP traffic is fully loaded, the power is allocated to maximize the VoIP capacity. Otherwise, it is allocated to maximize the data throughput. Finally, the performance of the proposed scheme is verified by computer simulation.