An　automatic　in　-situ　technique　for　thermal　resistance　measurement　based　on　an　electrical　method　is　proposed　in　this　paper.　In　contrast　to　the　conventional　thermal　resistance　measurement　circuits　of　the　electrical　method,　this　method　involves　direct　measurement　of　the　thermal　resistance　of　GaN　high-electron-mobility　transistor　(HEMT)　devices　in　an　operating　circuit　by　introducing　a　well-matched　radio-frequency　switching　circuit　into　the　operating　circuit.　The　advantage　of　this　method　is　that　the　thermal　resistance　can　be　measured　periodically　on　the　insitu　device　without　changing　the　electrical　characteristics　of　the　device.　Based　on　this　result,　the　cooling　curve　of　the　HEMT　device　in　the　power　amplifier　(PA)　circuit　is　collected　and　the　thermal　resistance　characteristics　of　the　HEMT　are　determined　via　the　structure　function　method.　The　purpose　of　this　paper　is　to　successfully　obtain　the　thermal　resistance　of　in　-situ　devices　through　an　automatic　in　-situ　measurement　circuit　of　thermal　resistance,　and　to　verify　the　feasibility　of　periodic　detection　of　thermal　resistance　of　in　-situ　devices.