We report the mechanism of current injection in (Ni/Au) /p -Alx Ga1-x N:Mg (0≤x<0.1) Ohmic contacts based on the temperature dependence of hole concentrations (p) and specific contact resistance (ρ c). The injection mechanism is found to be thermionic emission in all cases. A model is developed to describe the temperature dependences of p and ρ c for Mg concentrations from 1019 to 1020 cm-3. The model takes into account splitting in the valence band structure, hole activation energy, and Schottky barrier height. For GaN (AlGaN) these are found to be 132-140 (135-150) meV and 66-88 (84-93) meV, respectively.