We developed a high speed wireless LAN prototype in the 5GHz band. Maximum transmission rate of the developed WLAN system is 324Mbit/sec using 6 multi-channels of 802.1 la. We propose a closed-loop link adaptation scheme to improve MAC efficiency. The proposed scheme inserts MCS (Modulation and Coding Scheme) feedback information into Data frame and ACK frame. The MCS is calculated based on the estimated SNR. We compared the proposed link adaptation scheme with the legacy ACK-based link adaptation schemes by computerized simulation. The proposed link adaptation scheme achieved higher throughput than the legacy ACK-based link adaptation scheme. We also found that the proposed link adaptation scheme selected the optimal MCS on the developed WLAN system.