The luminescence properties of self-assembled CdS quantum dots (QDs) were studied. CdS QD structures formed on ZnSe buffer layers without capping layers showed intense photoluminescence (PL). The PL peak position could be controlled by changing the amount of CdS deposited. Introduction of the capping layer enhanced the PL intensity. The buffer layer material selected also affected the luminescence properties of the CdS QDs, and brighter PL with the higher transition energy was observed by replacing the ZnSe buffer layer with ZnSSe. A light emitting diode structure based on CdS QDs sandwiched in the pn junction of ZnSe was fabricated and bright blue or green luminescence from current injection was observed at 77 K as well as at room temperature.