The　junction-to-case　thermal　resistance　(R-theta　JC)　of　a　GaN/AlGaN　HEMT　is　measured　by　Transient　Dual　Interface　Method　(TDIM).　Different　from　other　works　about　TDIM,　an　improved　transient　infrared　microscope　is　used　to　measure　the　cooling　curves,　other　than　the　traditional　electrical　method.　Z(th)　curves　are　used　to　determine　the　R-theta　JC　following　the　procedure　of　JESD51-14.　The　results　demonstrate　that　the　R-theta　JC　at　40　W　power　dissipation　are　about　0.791　K/W.　In　order　to　validate　the　method,　measurements　following　MIL　Std　833　have　been　done,　and　the　results　are　consistent　with　the　existing　papers.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.