The　long　term　thermal　stability　of　the　die　attach　is　a　crucial　issue　for　high　brightness　light　emitting　diodes　(HB　LEDs),　which　affects　the　junction-to-case　thermal　resistance,　luminous　flux　and　life　time　seriously.　In　this　paper,　an　improved　power　and　temperature　cycling　method　is　proposed　to　evaluate　the　thermal　stability　of　the　die　attach　materials　for　HB　LEDs.　The　structure　function　method　is　adopted　to　monitor　the　degradation　of　the　die　attach　level　thermal　resistance　during　the　cycling　process　instead　of　the　traditional　junction-to-case　thermal　resistance　measurement,　which　provides　more　accurate,　quick　and　intuitive　results.　The　experimental　results　indicate　that　the　forming　of　solder　voids　is　the　main　degradation　mechanism　of　the　die　attach　for　HB　LEDs,　which　is　also　supported　by　the　scan　acoustic　microscope　(C-SAM)　measurement.　Comparing　thermal　stability　of　different　die　attach　materials,　Au/Sn　eutectic　soldered　LED　samples　present　better　performance　than　Ag　paste　soldered　samples　in　this　experiment.