Water-soluble mercaptoacetic acid-coated 3.1 nm CdS quantum dots (QDs) with two concentrations were selected for studying the correlation between the photoluminescence and the crystal growth mechanism. By achieving the classic Ostwald ripening mechanism and oriented attachment (OA) growth mechanism, we have shown that the evolution of the emission spectra were obviously different. The change in both the surface and internal defects during OA crystal growth were responsible for the specific variation of the photoluminescence of CdS QDs. Strategies for obtaining QDs with different luminescent properties are suggested.