Nanocrystalline CdS thin lms were deposited on glass substrates by chemical bath deposition (CBD). CdS thin lms with thickness of about 2900 A were annealed in a N2 atmosphere at several temperatures from 100 to 500 C for 60 min. The in uence of annealing on the structural and the optical properties was investigated by using eld-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), X-ray di raction (XRD), optical transmission and Fourier transform infrared spectroscopy (FTIR) measurements. The crystal structure of the CdS thin lm annealed at 200 C changed from the unstable cubic phase to the stable hexagonal phase. The crystallite size of the annealed CdS thin lms is estimated from the XRD peak by using the Scherrer formula. When the annealing temperature was increased above 200 C, the crystallite size of the annealed CdS thin lms increases and the dislocation density decreases. Also, the band gap of the annealed CdS thin lms changes with increasing annealing temperature. The broad absorption band of the water molecules decreases with increasing annealing temperature due to the release of the oxygen species.