The optical constants (n,k) of cadmium sulfide (CdS) thin films were determined in the spectral range of 0.55 μm to 1.80 μm from the optical absorption and transmittance measurements. The optical band gap (Eg) was determined for the films deposited at different substrate temperatures. Scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and X-ray diffraction (XRD) techniques were used to determine the morphology, composition, crystalline structure and crystallite size of the films. Evaporated CdS films show a predominant hexagonal phase with small crystallites. The optical band gap values of the films varied from 2.38 to 2.41 eV depending on the substrate temperature. It has been observed that the band gap and refractive index of the films have a close relationship with the size of the crystallites. The lower estimated value of band gap of the films is explained considering the effects of excitons and (or) some impurities.