We　presented　the　analysis　of　the　incomplete　conduction　in　bonding　medium　in　high　power　GaN-based　light-emitting　diode　(LED)　packages.　A　numerical　study　was　carried　out　with　parametric　model　to　understand　the　junction　temperature　variation　due　to　bonding　medium　defects.　Transient　thermal　measurement　was　performed　to　evaluate　LED＇s　junction　temperature.　Thermal　resistance　from　chip　to　lead　frame　was　20　K/W　in　our　sample　LED.　It　was　suggested　that　only　60%　of　the　surface　area　of　the　bonding　medium　was　involved　in　the　thermal　conduction.　This　result　was　also　supported　by　the　SEM　image.　Blocking　of　thermal　path　induced　by　ineffective　area　of　the　bonding　medium　was　regarded　as　a　factor　of　its　thermal　resistance.　Thus,　the　effective　area　of　the　bonding　medium　should　be　included　in　the　FEM　model　and　considered　as　another　important　factor　in　high　power　LED＇s　thermal　management.