Due to limited spectrum and increasing demand for wireless application, efficient use of wireless channel is becoming more important. In this paper, we propose a new approach of adaptive modulation to be employed in adaptive beamforming system to increase spectral efficiency while maintaining transmission quality. With a guarantee of bit error rate (BER) =10^-2 and 10^-4, our proposed method can achieve average throughput of 4bits/symbol at signal-to-noise ratio (SNR) = 16dB and 22dB, respectively.