Optical properties of Zn-doped CuInS2 thin films grown by double source thermal evaporation method have been studied. The amount of the Zn source was determined to be 0%-4% molecular weight compared with CuInS2 source. After that, samples were annealed in vacuum at the temperature of 450 degrees C in quartz tube. The optical constants of the deposited films were obtained from the analysis of the experimental recorded transmission and reflexion spectral data over the wavelength range 300-1800 nm. It is observed that there is an increase in optical band gap with increasing Zn % molecular weight. It has been found that the refractive index and extinction coefficient are dependent on Zn incorporation. The complex dielectric constants of Zn-doped CuInS2 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 according to the model of Spitzer and Fan. (C) 2011 Elsevier B.V. All rights reserved.