ZnSe thin films prepared on substrate by electrochemical method were deposited at and further irradiated with gamma irradiation of Co-60 source. The optical properties of the films are investigated using UV-VIS measurements. Absorbance of the films are observed to reduce after irradiation with γrays of different dose. The wavelength of the optical absorbance (A) decreases as the irradiation dose increases [A(400Gy) > A(300Gy) > A(250Gy) > A(200Gy) > A(0Gy)]. Other Optical properties like Transmittance, Optical band gap, reflectance, absorption coefficient, extinction coefficient and refractive index are determined from the absorbance result. The transmittance increases while the absorption coefficient decreases with increase in wavelength. The energy band gap of the sample decreases as the irradiation doses/fluence increases (0Gy = 3.02eV, 200Gy = 2.68eV, 250Gy = 2.49eV, 300Gy = 2.25eV and 400Gy = 2.0eV). This shows that the increase in gamma irradiation doses (Co-60 source) reduced the band gaps of ZnSe thin films and hence modifies the other physical properties.