The IEEE 802.11 MAC provides a method for multiple users to share the wireless media. The channel utilization is limited because of protocol overhead no matter how fast the payload is transmitted. To increase utilization, frame aggregation is a straightforward way to improve the channel efficiency. This paper presents an analytic method to estimate the utilization for RTS/CTS-based contention access mechanism with unidirectional or bidirectional aggregation. Simulation results are also provided to show the accuracy of the analytic model.