The different compositions of the ternary alloyed CdSexS1-x quantum dots (QDs) and CdSexS1-x/ZnS core/shell quantum dots (CSQDs) have been synthesized by the chemical routes. The radii of these QDs were determined by transmission electron microscope (TEM). The optical properties of these QDs were investigated by the absorption and fluorescent measurement. It was found that the absorption and fluorescent emissions were tuned by the component ratio, and the Commission Internationale de l′Eclairage (CIE) coordinates of the fluorescent spectra also depended on the composition. Compared with the CdSexS1-x QDs, the CdSexS1-x/ZnS CSQDs exhibit the fluorescence enhancement due to the surface passivation by shell coating. The composition-tuned optical properties may allow them to be used as fluorescent markers in biological imaging and to fabricate multicolor light emitting diode (LED).