Adaptive modulation has been shown to be effective in performance enhancement for multi-carrier wireless communication. In this paper we present an adaptive modulation framework for the OFDM based IEEE 802.11a wireless LAN system. The goal is to increase the data transmission rate subject to fixed transmission power and the upper bound constraint of packet error rate. Two adaptive modulation schemes are proposed and both use the estimated subband SNR values to adjust the modulation scheme in a blockwise manner. The simulation results indicate both schemes can effectively increase the average data rate up to 147% under the 802.11a model. The performance improvement is even prominent for severe frequency selective fading environments.