AlGaN/GaN high electron mobility transistor (HEMT) has many attractive material properties, such as high breakdown field, wide bandgap, and high thermal stability, which make it suitable for power electronic applications. This work presents thermal resistance constitution evaluation and experiments for packaged AlGaN/GaN HEMT. The investigation of thermal resistance is based on the closed-form expression heat transfer model which assumed that the device is viewed as a two-layer with a long and thin heat source on the top and an isothermal base at the bottom. The AlGaN/GaN device is divided into three distinct regions. Region I is contained in the GaN buffer layer. Regions II and III are contained in the substrate material. The thermal resistances of the packaged multi-finger AlGaN/GaN HEMT with 700 m Si substrate are 11.99 K/W and 122.43 K/W respectively, which provides a non-invasive method to evaluate the chip-level thermal resistance of AlGaN/GaN HEMTs. The validity of module is verified by comparing it with experimental observations. IR thermography microscope is utilized to measure the temperature of the active region with different driving conditions. The simulation process is calibrated against measurement data of a real device and delivers good predictive results for the DC characteristics.