This work presents thermal resistance constitution evaluation and electrical experiments for AlGaN/GaN HEMTs packaging device. The investigation of thermal resistance is based on the closed-form expression heat transfer model. Actual multi-fingers structure and thermal resistance was modeled. The Infrared thermography is utilized to observe heat distribution of the packaged GaN device by measuring the temperature of the active region in the operation. The simulation result is verified by comparing with experimental observations. The total thermal resistance is 134.42 K/W by calculation while that is 145.77 K/W by simulation. The self-heating effect under switching operation is verified by IR image. The simulation process is calibrated against measurement data of a actual device and provides good predictive results for the DC characteristics.