CdS films were prepared by R.F. magnetron sputtering method. The effects of vapor CdCl2 treatment on the properties of CdS films were studied. The vapor CdCl2 thermal treatment at different temperature was employed in a CSS device, using CdCl2 powder as evaporant. The structural and optical properties of CdS films were investigated by x-ray diffraction (XRD), PL spectra, and transmittance spectra, respectively. The results revealed that the CdS films had a structure of hexagonal wurtzite with a preferential orientation of the (002) plane. A better crystal quality and larger grain size, which are good for the solar cell application, were observed in the CdS samples annealed with CdCl2Subscript text.