The　junction　temperature　is　a　main　factor　affecting　the　device　performance　and　reliability.　The　thermal　resistance　is　usually　used　to　calculate　the　junction　temperature.　However,　the　thermal　resistance　is　not　constant　under　different　operating　conditions.　In　this　work,　we　examine　the　high-speed　electron　mobility　transistor　(HEMT)　from　the　CREE　Company　to　investigate　its　thermal　resistances　under　different　case　temperatures　and　dissipation　powers.　To　avoid　the　self-oscillating　phenomenon　of　the　HEMT　device,　a　circuit　is　designed　to　prevent　the　self-oscillating　in　experiment.　First,　the　temperatures　of　the　active　region　of　the　GaN　HEMT　device　are　measured　by　the　infrared　image　method　under　different　dissipation　powers　(including　2.8,　5.6,　8.4,　11.2,　and　14　W)　and　different　case　temperatures,　respectively.　Then　according　to　the　result　of　infrared　image　method,　the　simulation　model　is　set　up　by　using　the　Sentaurus　TCAD.　From　the　final　optimized　model,　we　extract　the　device　junction　temperature　and　calculate　the　thermal　resistance.　It　is　expected　to　ascertain　the　characteristic　of　the　thermal　resistance　and　compare　it　with　the　result　from　the　infrared　image　method.　It　is　found　that　as　the　device　case　temperature　increases　from　80　degrees　C　to　130　degrees　C,　the　thermal　resistance　changes　from　5.9　degrees　C/W　to　6.8　degrees　C/W,　i.e.,　it　is　increased　by　15%.　When　the　power　increases　from　2.8　W　to　14　W,　the　thermal　resistance　changes　from　5.3　degrees　C/W　to　6.5　degrees　C/W,　i.e.,　it　is　increased　by　22%.　This　phenomenon　is　mainly　attributed　to　the　changes　of　the　thermal　conductivity　of　device　materials.　According　to　the　formula　for　the　coefficient　of　the　thermal　conductivity　of　nonmetallic　material　SiC,　the　phonon　scattering　rate　becomes　larger　with　the　increase　of　temperature.　Thus,　the　phonon　mean　free　path　can　decrease　by　reducing　the　average　freedom　time.　Finally,　the　coefficient　of　thermal　conductivity　becomes　smaller.　It　was　reported　by　Kotchetkov　et　al.　(Kotchetkov　D,　Zou　J,　Balandin　A　A,　Florescu　D　I　2001　Appl.　Phys.　Lett.　79　4316)　that　the　coefficient　of　thermal　conductivity　of　GaN　becomes　smaller　under　high　temperature.　All　of　these　have　an　effect　on　the　heat　dissipation　of　the　device,　which　will　cause　the　thermal　resistance　to　increase.　Based　on　the　result　from　the　infrared　image　method　and　TCAD　simulation,　the　changing　characteristic　of　the　thermal　resistance　is　obtained,　thereby　reducing　the　errors　in　the　calculation　of　the　junction　temperature.