The carrier transport mechanism of Mg/Au ohmic contact for lightly doped β-Ga₂O₃ is investigated. An excellent ohmic contact has been achieved when the sample was annealed at 400 °C and the specific contact resistance is 4.3 × 10^-4 Ω·cm². For the annealed sample, the temperature dependence of specific contact resistance is studied in the range from 300 to 375 K. The specific contact resistance is decreased from 4.3 × 10^-4 to 1.59 × 10^-4 Ω·cm² with an increase of test temperature. As combination with the judge of E₀₀, the basic mechanism of current transport is dominant by thermionic emission theory. The effective barrier height between Mg/Au and β-Ga₂O₃ is evaluated to be 0.1 eV for annealed sample by fitting experimental data with thermionic emission model.