This　paper　investigated　the　rule　and　mechanism　of　thermal　resistance　varies　with　the　size　of　heat　source　in　GaN-based　high-electron　mobility　transistors　(HEMTs).　The　equation　of　heat　conduction　was　used　to　derive　calculate　thermal　resistance　which　increased　rapidly　with　the　size　of　heat　source.　To　validate　the　formula,　the　method　of　IR　thermal　imaging　combined　with　simulation　was　used　to　measure　junction　temperature　under　different　gate　length.　The　changing　mechanism　is　explained　by　two　views　of　isothermal　surface　area　and　thermal　capacitance.　This　analysis　is　useful　for　further　discussion　on　HEMT　failure　mechanisms　under　smaller　heat　source　sizes.　©　2017　IEEE.