CuInS2/ZnS(CIS/ZnS) core/shell quantum dots(QDs) were used in light emitting diodes (LEDs) as a wavelength converter. When the CIS/ZnS QDs in silicone encapsulants were applied to LEDs, they exhibited significant fluorescence instability followed by emissive blue-shift of 10 nm and decrease of luminescent intensities to 12% of its initial value within 2 h. To analyze this phenomenon, QDs dispersed in hexane were applied to LEDs, confirming that detachment of surface ligands and oxidation of QDs are main reason for degradation. At last, by using epoxy encapsulant which has much higher gas-barrier property than silicone, photostability of QDs is highly enhanced. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3610644] All rights reserved.