A method of performing contention-based uplink OFDMA transmission is proposed in accordance with one novel aspect. A wireless communications station (an AP) reserves both dedicated resource and contention resource for uplink OFDMA operation for a list of communications devices (STAs). For contention-based random access, the AP does not need to collect traffic requests from the STAs. The AP only needs to make simple resource arrangement. The AP only needs to specify the allocated resource for random access and the uplink OFDMA operation duration and timing for each uplink OFDMA packet. Each STA having traffic request will contend the resource based on a random access probability scheme.