Thermal effects were investigated to get a better understanding on the role of self-heating effects on the electrical characteristics of AlGaN/GaN HEMTs. This is implemented by solving simultaneously the acoustic and optical phonon energy balance equations and also takes into account the coupling of the two subsystems. The electro-thermal device simulator was used to observe the temperature profiles across the device. Hot spots or regions of higher temperatures were found along the channel in the gate-drain spacing. These preliminary results from the electro-thermal simulations suggest that the thermal effects do not have a drastic impact on the electrical characteristics, the current reduction falls between 5-10% over the simulated range of voltages. However, the non-equilibrium phonon effects might play an important role in determining the thermal distribution in these HEMTs and thus, resulting in reliability issues such as current collapse.