The Mg acceptor activation mechanism and hole transport characteristics in AlGaN alloy with Mg doping concen-tration (～1020 cm-3) grown by metal-organic chemical vapor deposition (MOCVD) are systematically studied throughoptical and electrical properties. Emission lines of shallow oxygen donors and (VIII complex)1-as well as V3N+and neu-tral Mg acceptors are observed, which indicate that self-compensation is occurred in Mg-doped AlGaN at highly doping levels. The fitting of the temperature-dependent Hall effect data shows that the acceptor activation energy values in Mg-doped Alx Ga1-x N (x=0.23, 0.35) are 172 meV and 242 meV, and the hole concentrations at room temperature are 1.2×1018 cm-3 and 3.3× 1017 cm-3, respectively. Therefore, it is believed that there exists the combined effect of the Coulomb potentials of the dopants and screening of the Coulomb potentials by a high hole concentration. Moreover, due to the high ionized acceptors’ concentration and compensation ratio, the impurity conduction becomes more prominent and the valence band mobility drops sharply at low temperature.