Abstract: Water soluble quantum dots (QD′s) of CdS with rich S2- surface and having good fluorescence stability, were prepared by the slow reaction of CdCl2 and Na2S solution with controlled and low concentrations in an ice bath, using sodium citrate and sodium hexametaphosphate as stabilizing agents. Reaction between the CdS QD′s and metronidazole (MTDZ) was studied by fluorescence spectrometry, UV-VIS-absorption spectrometry and resonance scattering spectrometry. It was shown that quenching of fluoresence emission of the CdS QD′s was due to the binding of MTDZ with the excessive S2- on the surface of the CdS QD′s, and that the fluorescence quenching was attributed to static quenching, resulting mainly from combination by electrostatic force.