This paper proposes a new mathematical analysis model for computing the average packet delay of IEEE 802.11 protocol. This analysis is valid by comparison with the simulation result based on OPNET. The proposed analysis verifies that when RTS/CTS mechanism achieves lower packet delay, with respect to the basic access mechanism, it is of great importance to the rational use of RTS/CTS mechanism and the notable increase in the performance of IEEE 802.11 WLAN.