Since the condition of wireless channel is very complicated, many rate adaptation mechanisms were proposed to enhance the IEEE 802.11 systems, but these may cause multiple rates in a single network. IEEE 802.11 supports adaptive rate mechanism, which selects the transmission rate according to the condition of wireless channel, to enhance the system performance. However, IEEE 802.11 faces the performance anomaly in the multi-rate network. Thus the analysis of multi-rate MAC performance and protocol design under the multi-rate environment are very important. In this paper, we first study the performance of multi-rate IEEE 802.11 MAC protocol both under the basic access and RTS/CTS access which is determined by the RTS Threshold (if the size of data packet is greater than RTS Threshold, the node works in the basic access mode. Otherwise, the node works in the RTS/CTS access mode). Then we study the impact of RTS Threshold on the network throughput under various scenarios. Simulation results show that the number of node with higher rate is relatively large or the number of node is relatively small, the RTS Threshold should increase to improve the network throughput. This conclusion indicates that we can adjust the RTS Threshold to improve the network throughput in the multi-rate network.