Neutral red (NR) is polycrystalline in powder form, it transforms to nanocrystallite phase upon thermal deposition. Gamma-ray irradiation with doses 1.25-6 KGy induced partial transformation of nanocrystallite phase to amorphous structure. The changes of optical constants with gamma-ray doses were calculated using spectrophotometer measurements of transmittance and reflectance at normal incidence of light over spectral range 200-2500 nm. The complex refractive index of NR film is highly influenced by exposure to gamma-ray irradiation, the onset and optical energy gaps decrease with increasing gamma-ray doses, and Urbach tail increases linearly with increasing irradiation dose. The type of electronic transition, oscillator, and electric dipole strengths and dispersion parameters were determined before and after irradiation. The spectral behavior of dielectric constant with gamma-ray doses was also estimated.