In　the　article,　a　new　thermal　resistance　measurement　method　for　GaN　high　electron　mobility　transistor　(HEMTs)　is　proposed　based　on　temperature　sensitive　electrical　parameter　(TSEP)　with　the　use　of　MOS　switches　to　connect　the　drain-source　at　the　beginning　of　the　measurement　processes.　Compared　with　traditional　methods,　the　connection　of　the　source　and　drain　reduces　the　delay　caused　by　the　matching　circuit.　In　our　experiments,　we　measured　a　GaN　HEMT　at　48　V　with　a　power　consumption　of　5.28　W.　The　delay　time　was　less　than　2　mu　s,　and　the　thermal　resistance　was　9.85　degrees　C/W.　We　also　measured　the　thermal　resistance　at　the　same　current-different　voltages　and　different　voltages-same　power.　The　results　show　that　the　thermal　resistance　of　the　GaN　HEMT　increases　with　the　increase　of　the　power　and　voltage.　Furthermore,　the　results　have　been　verified　by　infrared　(IR)　and　simulation　methods.　Compared　with　Raman　thermography　and　IR,　this　method　can　conveniently　measure　the　thermal　resistance　of　GaN　HEMTs　without　breaking　the　package,　and　the　temperature　accuracy　was　+/-　2　degrees　C　or　better.　And　the　thermal　resistance　information　of　each　layer　of　structure,　such　as　the　chip-solder-carrier,　can　be　extracted　by　structural　function　method　to　improve　the　thermal　design　of　GaN　HEMTs.