Thermal　transient　measurement　of　the　808　nm　GaAs-Based　laser　diodes　(LDs)　before　and　after　the　constant　current　stress　are　presented　and　discussed　in　this　paper.　Aging　tests　are　carried　out　under　the　conditions　of　the　constant　current　stress　(1　A)　for　255　hours.　The　total　thermal　resistance　increases　from　7.0　to　8.8　°C/W　before　and　after　degradation.　Furthermore,　the　contribution　of　each　component　to　the　total　thermal　resistance　has　been　obtained　from　the　differential　structure　function　and　cumulative　structure　function　before　and　after　stress.　The　thermal　resistance　of　chip　maintain　almost　constant　before　and　after　stress,　while　significant　increase　in　the　thermal　resistance　of　solder　layer　(indium　attaching　material)　and　package　body　are　observed　after　degradation.　These　results　indicate　that　the　thermal　properties　of　solder　layer　and　package　body　degrade　apparently　as　contrast　to　the　chip　of　LDs.　The　thermal　properties　of　solder　layer　and　package　body　have　critical　effects　on　the　performance　of　LDs.　Thus　the　performance　of　LDs　could　be　improved　through　optimizing　of　solder　material　and　package　body.　©2010　IEEE.