We report best dielectric function of ZnS by spectroscopic ellipsometry in the 3.7 - 6.0 eV photon energy range at room temperature. Using proper wet chemical etching procedure, natural overlayer was removed to obtain the pure dielectric function of ZnS, which had a higher <$xi$$_2$> value at the El band gap peak than that previously reported. We also determined the dielectric property of the natural overlayer on ZnS by following the evolution of <$xi$$_2$> with chemical etching. We found that the optical property of the overlayer was well described by amorphous semiconductor model.