In　this　paper,　we　performed　a　high-resolution　measurement　of　channel　temperature　rise　in　GaN-based　high　electron　mobility　transistors　(HEMTs).　Cathodoluminescence　spectroscopy　in　the　scanning　electron　microscope　was　used　to　probe　the　temperature　rise　with　several　tens　of　nanometers　spatial　resolution　and　accuracy　better　than　+/-　8　degrees　C.　We　also　determined　the　temperature　distribution　and　peak　temperature　change　with　the　power　density　in　active　AlGaN/GaN　HEMTs　in　the　source-gate　and　gate-drain　openings.　The　measured　results　agree　reasonably　well　with　physical　2D　electrothermal　simulations　and　Raman　thermography.