Abstract: Self-heating is a very severe factor for GaN HEMT performance. In this paper, we comparative study thermal characteristics of the conventional packaging and flip-chip GaN HEMT devices with various substrates and improve heat dissipation of the flip-chip devices by using carbon nanotubes with high thermal conductivity. From the results, flip-chip lower temperature of device, while the carbon nanotubes increase the heat dissipation of flip-chip. The thermal resistance of devices with sapphire, Si and SiC substrates decreased by 77.1%, 50.6% and 32.9%, respectively, with carbon nanotubes. Therefore, carbon nanotubes can improve the heat dissipation of the flip-chip structure and enhance the performance and reliability of the device.