AlGaN/GaN high electron mobility transistors (HEMTs) are one of the prospective candidates for high switching frequency power electronics applications thanks to its wide band gap (3.4eV), high breakdown voltage, large critical electric field, high carrier mobility, and the inherent high speed characteristics. With the high power densities that AlGaN/GaN HEMTs are capable of reaching, heat dissipation is a crucial issue. This paper presents the thermal analysis of packaging development, the device is attached on a V-groove copper base, and mounted on TO-3P leadframe to enhance Si substrate thermal dissipation. The effects of structure design and fabrication processes on the device performance as well as thermal resistance were studied. In addition, micro-Raman/Infrared (IR) thermography was used to investigate temperature profiles and hot spot of the devices. Simulations provide key insights into device operation and the thermal mechanisms that affect reliability. After incorporating self-heating effect in calculations of current-voltage characteristics, our results agreed well with experimental data.