Recently, various types of quantum dots (QDs) have been synthesized and widely used in optoelectronic devices and biomedical researches. Alloyed QDs, which are well-known for their superior optical properties such as fluorescence stability and high photoluminescence intensity, have been synthesized with combination of various starting materials. The band gap energy of semiconductor can be controlled through change of the ingredient and the inner structure. In this work, we have synthesized CdSeS alloyed QDs by using one pot reaction with coordinating solvent. Cadmium oxide (CdO), selenium powder (Se), and sulfur powder (S) were used as precursor for the preparation of alloyed QDs. The typical size of the QDs was about 3.3 nm. With the S content decreased, the fluorescence spectra were red-shifted, and fluorescence lifetime was decreased.