Accurate prediction of resistances at the transistor’s access zone and their temperature dependency are very important, since the current flow to the device depends on that region. In this paper, we have modeled the access resistances and have verified their temperature dependency with the experimental results in the case of a 0.25 μm gate GaN HEMT. At first, the carrier concentration profile and its mobility associated with the two-dimensional electron gas (2-DEG) were evaluated to model these resistances. Also, we have measured the threshold voltage and have studied its temperature dependence in this estimation process. Apart from that, thermal resistance and its change with temperature were investigated, as it is an important factor in describing the 2-DEG transport mechanism. We also estimated the variation of channel temperature for nine different temperatures (from 233 to 423 K) and validated the output with the measured data. The studied results can be used for the prediction of GaN HEMTs characteristics with temperature variation and can be included in the simulation packages.