GaN HEMT reliability evaluation in a typical Arrhenius manner requires establishing peak junction temperature for a particular stress condition. Several new techniques have yielded promising results toward establishing peak temperature for these devices in combination with detailed physical modeling, particularly micro-Raman imaging. This paper compares results from finite element modeling to measurements by, infrared imaging and micro-Raman imaging. The limitations of IR imaging were confirmed similar to earlierreports. Two techniques for establishing temperature from micro-Raman measurements were used to reveal excellent correlation to the model, and also provide insight into the relationship between temperature and structural change in the, device. Temperature modeling data is reported for base plate temperature from 85 degrees C to 250 degrees C for practical GaN HEMT devices. Implications of the measurements for GaN HEMT reliability stress testing and analysis will be disscussed.