Thermal　characteristic　of　light　emitting　diodes　is　one　of　primary　reliability　parameters.　In　this　paper,　the　thermal　resistances　of　different　kinds　of　blue　and　white　high　power　LEDs　are　measured　by　the　forward　voltage　based　method.　The　relationships　between　thermal　resistance　and　heat　time　at　measurement　currents　1mA,　5mA　and　10mA　are　obtained　respectively.　The　results　show　that　there　is　optimum　heat　balance　time　of　300~1200s　to　measure　the　thermal　resistances　at　room　temperature.　The　influence　of　current　crowding,　the　phosphor　and　heat　sink　to　the　thermal　resistance　has　been　demonstrated.