Abstract: The paper proposes a new MAC protocol to improve the capacity of 802.11b. IEEE 802.11b provides very high raw bandwidth. However, the overheads introduced by the physical and MAC layers are also increasingly substantial. Moreover, there are two kinds of PLCP (physical layer convergence protocol) PPDU (PLCP protocol data unit), but many researchers did not take full advantage of short PLCP PPDU to improve the capacity of 802.11b. A new physical-adaptation algorithm is proposed, which is able to adaptively choose long or short PLCP PPDU and compute network allocation vector (NAV). The proposed MAC protocol and 802.11b were evaluated by NS. Performance results reveal that the proposed MAC protocol performs better than the 802.11b in term of average end-to-end delay, goodput and average successful delivered rate, specially for high load and high raw bandwidth.