The CdS thin films were deposited on glass substrates by two different methods, chemical bath deposition (CBD) and thermal evaporation (T.E.). The effect of annealing temperature on the structural and optical properties of the CdS thin films were investigated. The structural properties of CdS thin film was studied by X-ray diffraction. It was observed that the films have a polycrystalline hexagonal (wurtzite) structure with preferred orientation along (002) plane. The crystallite size calculated from XRD increases as the annealing temperature is increased. We observed that the lattice constant, micro strain, dislocation densities and number of crystallites per unit area of the films are quite different in these processes from XRD analysis by considering high intense diffraction peaks of the as-deposited and annealed films. The energy band gap in nanocrystalline CdS thin films has been estimated from absorption measurements. The band gap values for CBD-CdS thin films decreased from 2.75 to 2.45 eV and for T.E.-CdS thin films decreased from 2.60 to 2.43 eV with increasing annealing temperature. Small nanocrystals display wide band gaps as a result of the quantum confinement experienced by nanocrystals of a certain size

CBD --- thermal evaporation --- CdS thin films --- annealing temperature --- structural and optical properties