Fluorescent semiconductor nanocrystals have been widely used as fluorescent materials in chemical sensors, biotechnology, medical diagnostics, biological imaging and many other fields. Compared to the conventional organic fluorophores, the inorganic quantum dots (QDs) have many advantages, including broad absorption spectra, narrow emission spectra, good photostability and long fluorescent lifetime after excitation. Here, the high quality CdS QDs were synthesized directly from sulfur and CdO using the paraffin liquid as solvent and the oleic acid as the reacting media. The synthesized CdS QDs with a zinc blende (cubic) crystal structure were proved by X-ray diffraction. HRTEM observation revealed that the CdS QDs were uniform and the average grain size was about 4 nm. The optical properties of the CdS QDs were characterized by using photoluminescence (PL) spectrophotometer and Ultraviolet-visible (UV-Vis) absorption spectrophotometer. The formation mechanism of CdS QDs in the paraffin liquid and oleic acid system was proposed.