CdS nanoparticles with high uniformity have been synthesized through a new liquid-liquid approach, using dimethyl sulfoxide and the mixture of oleyl amine and oleic acid as separate solvents, more convenient inorganic cadmium salt as cadmium source, and sulfur powder as sulfur source. The process can be conducted under mild conditions. It is found that the initial Cd/S molar ratio of precursor plays important role in controlling particle size. Higher concentration of sulfur precursor results in the larger particle size. The obtained colloidal CdS nanocrystals show obvious absorption and fluorescence in visible light region. When using methylene blue as degradation agent, the obtained CdS nanoparticles exhibit high photocatalytic ability.