Optical properties of In2S3 thin films grown by vacuum thermal evaporation method have been studied as function of Na incorporated In2S3 films. The optical constants of the deposited films were obtained from the analysis of the experimental recorded transmission and reflectance spectral data over the wavelength range of 300-1800 nm. It has been found that the refractive index and extinction coefficient are dependent on the Na incorporated In2S3 films annealed under nitrogen at 300 degrees C for 2 h. Using the Wemple-Didomenico model, we calculated the static refractive index n(lambda), the oscillation energy gap E-0 and the dispersion energy E-d. The dispersion energy changes very slowly as function of at.% Na. The complex dielectric constants of In2S3 films have been calculated in the investigated wavelength range. It was found that the refractive index dispersion data obeyed the single oscillator of the Wemple-Didomenico model, from which the dispersion parameters and the high-frequency dielectric constant were determined. The electric free carrier susceptibility and the carrier concentration on the effective mass ratio were estimated. The values of the electric permittivity are practically constant in the spectral field of the visible range. The values of the oscillation energy describe the expression E-0 = 1.5 Eg and check the Wemple-Didomenico model. (C) 2011 Elsevier B.V. All rights reserved.