Heat dissipation properties are very important in AlGaN/GaN RF high electron mobility transistor (HEMT) devices operating at high frequency and high power. Therefore, in order to extract the thermal conductivity of the substrate and device, which are essential for the analysis of the heat dissipation characteristics, various methods of extraction were attempted. And this experiments were conducted in parallel with micro-raman measurement and thermal simulation. As a result, it was possible to extract the thermal conductivity of each GaN-on-diamond epi layer by matching the thermal simulation data and the shift of the micro-raman peak according to various operating states and temperatures of the transmission line method (TLM) pattern. In particular, we tried to extract the thermal boundary resistance (TBR) of the interface layer (SiNx) for adhesion between GaN and diamond, which greatly affects the thermal conductivity of the device, and successfully extracted the following thermal conductivity value of KTBR = 3.162·(T/300)-0.8 (W/mK) from GaN and diamond interface layer.