Polycrystalline ZnSe thin films with thickness 720 nm were prepared by thermal evaporation under ultra vacuum onto glass substrates. The optical properties of the as deposited and UV-irradiated films with different exposure times (0, 30, 60, 90, and 120 min.) were reported. The polycrystalline nature of the films was detected by XRD measurements. XRD patterns for UV irradiated films show that the intensities of the peaks increase with increasing the UV-irradiated time but the full width at half maximum (FWHM) decreases. The optical constants of the films were calculated by Swanepoel's method in terms of wedge shape model to obtain both thickness and refractive index with high precision. The energy gap of ZnSe films was determined by using the transmission in terms of transmission spectra in the strong absorption region. In terms of the obtained results, the increase of exposure time improved shrinkage of non-uniform transmission in both the strong and medium absorption region of ZnSe film. The optimal improvement was at UV-irradiation time equal 120 min. Accordingly, the possibility of asserting that such tenability in the optical refractive index and energy gap of ZnSe thin films with UVirradiation serves as a promising film in optoelectronic devices.