The temperature dependence of the specific contact resistance in annealed Ti/Al/Ni/Au multilayers on (NH4)2Sx treated n-type GaN has been studied in the temperature range from 25 to 600 °C by the transmission line technique. It is found that the specific contact resistivity ρc of the sample treated with (NH4)2Sx solution for 5 min at 90 °C decreases with increasing measuring temperature, while the ρc of the sample treated with (NH4)2Sx solution for 25 min at 90 °C increases with increasing measuring temperature. Excellent agreement with the “5 min treated” sample can be obtained by the field emission model with an average Schottky barrier height (SBH) ΦB=1.05 eV. Meanwhile, a field emission model with a temperature-dependent effective SBH is suggested to be responsible for the “25 min treated” sample in which metal/semiconductor interface potential pinch-off may occur. The high-resolution transmission electron microscope results support the above model.