The optical constants and thickness of CdSe thin films prepared by quasi close-space sublimation method are determined. The structural investigations performed by means of XRD technique showed that the films have a polycrystalline and hexagonal structure. The optical constants and the band gap of the films under study have been determined (E-g= 1.68 eV). Optical properties (refractive indexn(lambda), absorption coefficient alpha(lambda), and extinction coefficientk(lambda)) of thin films and thicknessdcan be determined from the transmission spectrum. The dispersion of the refractive index was explained using a single oscillator model. Single oscillator energy and dispersion energy are obtained from fitting. Optical parameters of the films were determined using the Cauchy, Sellmeier and Wemple models.