The influence of annealing and γ-irradiation on the optical and structural parameters of Ge15Se50Te35 films synthesized by thermal evaporation technique was investigated. The samples were characterized using Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Differential Scanning Calorimetry (DSC), and Energy Dispersive X-ray Analysis (EDX) techniques. XRD analysis showed that the as-prepared films are amorphous, while the annealed ones are polycrystalline. The presence of the crystalline particles in the annealed films is attributed to Se and GeSe2 phases, which have a monoclinic and an orthorhombic lattice structure, respectively. The observation of glass transition, at 465 ± 1 K, and crystallization peaks, at 625 ± 1 K, in DSC curve confirms the glassy as well as the amorphous nature of the studied composition. On the other side, the analysis of the absorption spectra in the wavelength range (200–900 nm) of annealed and γ-irradiated Ge15Se50Te35 thin films reveals that the allowed indirect transitions are responsible for inter-band transitions. The indirect band gap of Ge15Se50Te35 films was found to decrease from 1.44 to 1.21 eV and from 1.44 to 1.27 eV with increasing the annealing temperature and irradiation dose, respectively. Other optical parameters such as the extinction coefficient and the refractive index were calculated, which depend on thermal annealing and γ-irradiation. These results are interpreted in terms of the structure defects formed in the investigated annealed and irradiated films.