Adaptive modulation is a very useful technique for enhancing data rate over a fading and shadowing channels. In this paper an adaptive M-ary Quadrature Amplitude Modulation (M-QAM) technique is presented for Nakagami-m-Gamma (NG) channel model. The technique presented provides a guaranteed level of bit error rate (BER) performance while at the same time optimizing the spectrum efficiency. Analytical expressions for probability density and cumulative distribution functions for the NG model are given and closed-form expressions for outage probability and average spectrum efficiency are derived conditioned on target BER. Numerical results show that the adaptive modulation technique presented improves spectrum efficiency over NG channel with acceptable BER performance.