The transmittance (T) of thin films of CdI2, prepared by thermal evaporation technique on quartz substrates, have been measured over the wavelength range 200−900 nm. From analysis of the transmittance data, the optical constants, the refractive index (n) and the extinction coefficient (k), have been studied. Analysis of the refractive index (n) yields a low frequency dielectric constant, average oscillator strength and average oscillator energy. From analysis of the absorption coefficient (α), the fundamental absorption edge can be determined. Both allowed direct transitions and allowed indirect transitions are observed. The composition of films is checked using energy dispersive X-ray (EDX) spectroscopy technique. X-ray diffraction (XRD) measurements showed that the CdI2 films evaporated at room temperature substrates were characterized by a polycrystalline form. At large thicknesses the films indicated the inhomogeneity. The effect of annealing temperature (up to 523 K) on the film properties has been studied.