GaN-based electronic devices have been demonstrated to be ideal for high power and high frequency applications. GaN-based devices fabricated on sapphire substrates have been known to suffer from serious heating issues due to their poor thermal conductivity. Thermal analysis for GaN-based devices has been increasingly sought after since their heat dissipation can sufficiently degrade the DC and the RF performances of the device, in particular during the high power operation. This paper focuses primarily on the thermal analysis of AlGaN/GaN heterostructure field-effect-transistors (HFETs). We present our thermal simulation and compare that with experimental results. Liquid crystal technique was utilized to map the thermal profile of the device surface.