Various thicknesses of cadmium sulfide ZnS thin films were evaporated onto glass substrates using the thermal evaporation technique. X-ray diffraction analysis indicates that both the film and powder have cubic zincblende structure. The microstructure parameters, crystallite size and microstrain were calculated. It was observed that the crystallite size increases but the microstrain decreases with increase the film thickness. The band gaps of the ZnS thin films were found to be direct allowed transitions and increase from 3.33 to 3.46 eV with increasing the film thickness. The refractive indices have been evaluated in transparent region using the envelope method in the transparent region. The refractive index can be interpolated and extrapolated in terms of Cauchy dispersion relationship over the whole spectra range, which extended from 300 to 2500 nm. It was observed that the refractive index, n increase on increasing the film thickness.