The channel characteristics of IEEE 802.11 WLAN vary with time and this can affect packet transmission performance. For achieving robust and efficient transmission, the transmission rate is controlled by exploiting the multi-rate capability of the IEEE 802.11 physical layer (PHY) to respond to the time-varying channel condition. In this paper, we propose a novel rate adaptation scheme, called RA-MCE, in which the transmitter estimates channel quality in the MAC layer to enhance throughput performance without the need to use the RTS-CTS mechanism nor to modify the IEEE 802.11 standard. RA-MCE adaptively controls the transmission rate according to the estimated channel quality by the MAC layer channel quality estimator (MCE) that uses only local MAC layer measurements. Through extensive simulations, we validate the accuracy of MCE and evaluate the performance of RA-MCE to show that it achieves higher throughput performance than other rate adaptation schemes under various circumstances.