Ge15Se75Zn10 thin films were deposited onto cleaned glass substrates by the vacuum thermal evaporation technique. The as-deposited and annealed thin films were characterized by scanning electron microscopy, X-ray diffraction and optical spectroscopy. The average particle sizes of the crystallized GeSe and ZnSe phases increase with increasing annealing temperature. Some optical parameters of the as-deposited and annealed Ge15Se75Zn10 thin films were studied using both transmittance T(λ) and reflectance R(λ) measurements. The analysis of T(λ) and R(λ) data revealed the existence of direct optical band gap (EG) and the corresponding values were calculated to be 2.913, 2.802 and 2.692 eV for the as-deposited and annealed thin films at 373 and 423 K, respectively. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple-Didomenico (WD) model. The width of band tails of localized states (Ee), the single oscillator energy (E0), the dispersion energy (Ed), the high frequency dielectric constant (εL) and the steepness parameter (S) were estimated.