Abstarct: CdS thin films have many applications such as in solar cells. In this thesis, CdS and Co doped CdS thin films were deposited on glass substrate by thermal evaporation method and their physical properties were investigated. Our experimental works were as following: At first, CdS thin films were prepared with thickness about 1.5 micrometers and at room temperature. X-ray diffraction results of showed that the films have hexagonal structure with preferred peak along (002) direction. Their transmittance and absorption spectra were also recorded by Uv–Vis spectrometer and then their optical constants were calculated. The measured band gap of the films was about 2.42 eV. In next step, Cd1-xCoxS thin films (x= 0, 0.025, 0.05, 0.1) with thickness about 50nm, 500nm were deposited on glass substrate with substrate temperatures of 27℃, 100℃ and 180℃. The X-ray diffraction spectra of the films revealed a little change in position and intensity of the (002) peak by changing the Co concentration, substrate temperature and thickness of the films. No extra reflections were observed due to the Co cluster. Grain size increased and lattice parameters decreased by increase in Co concentration. The other structure peaks were observed in spectra by increasing of the film`s thickness and substrate temperature. Scanning electron microscope was used to study the surface of the films. Band gap of the Co doped films were calculated by measuring the transmittance and the results indicated decrease of the band gap by increasing of cobalt concentration. For investigation of magnetic properties, the films were studied by alternating gradient field magnetometer, magneto – optical study (Kerr- and Faraday effect) but these existing systems were not able to detect the magnetic signals. Finally, new films were prepared by diffusion method. CdS thin films were deposited on glass substrate with thickness of about 194nm and then cobalt was deposited on the CdS films with thicknesses of about 8nm, 13nm and 19nm.Then the films were annealed at 400℃ in vacuum to diffuse the Co into the CdS matrix and their structure, optical and magnetic properties were studied as the others. But to investigate the magnetic properties of the films, hysteresis loops of the films were measured at room temperature by vibrating sample magnetometer. The results showed that the films have a mixed behavior of dia- and antiferromagnetism.