Several thin films of Te10Ge10Se77Sb3 chalcogenide glass of different thicknesses (250 nm to 400 nm) were prepared by thermalevaporation under vacuum of 133×10-6 Pa (10-6torr). X- ray diffraction analysis showed the amorphicity of the preparedfilms which become partially crystalline by annealing. Transmittance and reflectance measurements in the spectral range of200 nm to 2500 nm have been carried out at normal incidence. The analysis of the absorption coefficient data showed theexistence of indirect transition for the photon energy E in the range 1～3 eV and direct transition for E＞3 eV. From thedetermination of the optical constants (n, k), the dispersion of the refractive index has anomalous behaviour in the region ofthe fundamental absorption edge, and followed by the single- effective oscillator approach. The investigated optical parameterssuch as the optical energy gap Eopt, the high frequency dielectric constant εoo, the oscillator position λo, and the oscillatorstrength So, were significantly affected by the film thickness. The characteristic energy gap obtained from the conductivitymeasurements is nearly half the value of that obtained from the optical data as in the case of thickness 400 nm. The activationenergy is 0.65 eV and the indirect optical gap is 1.32 eV.