Epoxy molding compounds (EMC) with higher thermal stabilities are urgently needed as the light emitting diode (LED) becomes brighter and the wavelength of the its light becomes shorter. This paper proposes a simple reliable method of evaluating the thermal stabilities of commercial EMCs. The transmittances of most commercial EMC samples for high power short wavelength LED packages were decreased by heat treatment at 150oC for 200hr. Also the thermal stabilities of the samples were confirmed by measuring the weight losses through TGA. The experimental results suggest that employing a good heatsink is indispensable in highly bright short wavelength LED packages.