Zn-In-Se (ZIS) thin films were prepared by sequential evaporation of its elemental sources on the glass substrates. The effect of thermal annealing under nitrogen environment on the optical properties of the films was discussed. In addition to the comparative study of three different annealing temperatures, the results were analyzed by relating with their structural and compositional characteristics. The optical analyses were based on the observed interference fringes on their transmission spectra of the films. The refractive indices were calculated by means of envelope method (EM) and the continuity of the refractive indices was evaluated by three-term Cauchy fitting process. From the results of the refractive index calculations, the real and imaginary part of the dielectric constant were determined. The optical absorption coefficients of the films were found in the range of 10(3)-10(4) cm(-1) over the visible and near-infrared region and by using these values, the extinction coefficients were calculated. Moreover, the band gap values were calculated from the corresponding Tauc plots, and the refractive index dispersion over the measured wavelength range was investigated with single-oscillator model (SOM) and the related parameters were obtained. (C) 2017 Elsevier GmbH. All rights reserved.