We report the evolution of optical absorption properties of 800 keV Ar ion irradiated NiO thin films through UV-Vis characterization. Our results indicate the existence of both Mott-Hubbard (d → d transition) and charge-transfer (p → d transition) characteristic of NiO. The optical band gap of NiO increases from 3.58 to 3.75 eV when irradiated at the fluence of 5 × 1014 ions cm-2 but it does not show any remarkable variation upon 800 keV Ar ion irradiation at higher fluences. The refractive index and electron polarizability at different ion fluences have been determined from the optical band gap. Both refractive index and electron polarizability follow an opposite trend to that of the energy gap as a function of ion fluence.