CdS thin films have been prepared by chemical bath deposition (CBD) method on glass substrates at solution temperature 80 oC. Cadmium acetate was used as a source of Cd+2 and thiourea as a source S-2. After three times of sequences deposition a 100 nm CdS thin films is obtained. The Structure of these films studied by X-Ray diffraction were polycrystalline hexagonal phase. The films exhibited high optical transmittance T~50-100 % and low reflectance in the wavelength (500-900)nm which make them useful as a transparent window in solar cells. The optical constants (n,k) were also calculated in the same spectrum region. The optical transitions were due to direct allowed in quantum mechanical sense. The value of the optical energy gap for the as deposited and annealed films was in the range (2.35-2.43) eV. Vacuum annealing at (200,300)oC improved the crystalline structure of the films.