The junction temperature is a main factor affecting the device performance and reliability. The thermal resistance is usually used to calculate the junction temperature. However, the thermal resistance is not constant under different operating conditions. In this work, we examine the high-speed electron mobility transistor (HEMT) from the CREE Company to investigate its thermal resistances under different case temperatures and dissipation powers. To avoid the self-oscillating phenomenon of the HEMT device, a circuit is designed to prevent the self-oscillating in experiment. First, the temperatures of the active region of the GaN HEMT device are measured by the infrared image method under different dissipation powers (including 2.8, 5.6, 8.4, 11.2, and 14 W) and different case temperatures, respectively. Then according to the result of infrared image method, the simulation model is set up by using the Sentaurus TCAD. From the final optimized model, we extract the device junction temperature and calculate the thermal resistance. It is expected to ascertain the characteristic of the thermal resistance and compare it with the result from the infrared image method. It is found that as the device case temperature increases from 80 ℃ to 130 ℃, the thermal resistance changes from 5.9 ℃/W to 6.8 ℃/W, i.e., it is increased by 15%. When the power increases from 2.8 W to 14 W, the thermal resistance changes from 5.3 ℃/W to 6.5 ℃/W, i.e., it is increased by 22%. This phenomenon is mainly attributed to the changes of the thermal conductivity of device materials. According to the formula for the coefficient of the thermal conductivity of nonmetallic material SiC, the phonon scattering rate becomes larger with the increase of temperature. Thus, the phonon mean free path can decrease by reducing the average freedom time. Finally, the coefficient of thermal conductivity becomes smaller. It was reported by Kotchetkov et al. (Kotchetkov D, Zou J, Balandin A A, Florescu D I 2001 Appl. Phys. Lett. 79 4316) that the coefficient of thermal conductivity of GaN becomes smaller under high temperature. All of these have an effect on the heat dissipation of the device, which will cause the thermal resistance to increase. Based on the result from the infrared image method and TCAD simulation, the changing characteristic of the thermal resistance is obtained, thereby reducing the errors in the calculation of the junction temperature.