An initial sharp decrease in the light output of a high-power light-emitting diode is observed when it is exposed to humid condition. TGA and energy-dispersive system analyses confirm the possibility of moisture entrapment in the silicone encapsulation; moreover, the light scattering model verifies qualitatively the scattering of the light due to the entrapped moisture, and it is this scattering that renders an initial sharp drop in the light output. The finding shows the importance of pore size of the silicone gel in order to prevent such sharp decrease in the light output under a humid environment.