Chalcogenide Se75Sb10In15 thin films of different thickness (50–300 nm) are deposited using thermal evaporation technique. The thermogram of the chalcogenide bulk Se75Sb10In15 is obtained using a differential scanning calorimetry (DSC). The crystallization temperature Tc, peak crystallization temperature Tp and melting temperature Tm, are identified. The X-ray diffraction (XRD) examination indicates the crystallinity of the as-deposited film decreases with increasing of thickness. Optical transmission and reflection spectra are recorded in the wavelength range of the incident photons from 250 to 2500 nm. It is found that the film thickness affects the absorption coefficient, refractive index, extinction coefficient and the width of the tails of localized states in the gap region. The absorption mechanism of the as-deposited films is a direct allowed transition. The optical band gap energy (Eg) decreases from 3.31 to 2.51 eV with increasing the film thickness from 50 to 300 nm. The behavior of Eg is explained on the basis of the structure disorders in the thicker films. The effect of the film thickness on the single-oscillator and dispersion energies is studied by the dispersion analyses of the refractive index.