Annealing effect on the structural and optical properties of the quaternary Cu-Ag-In-Se thin film deposited by the thermal evaporation has been investigated. The evaporation source was prepared by using vertical Bridgman-Stockbarger crystal growth system. Structural analysis indicated that annealing the films following to the deposition resulted in the changes from amorphous to polycrystalline phase with the preferred orientation along (1 1 2) direction. In order to determine the optical properties of the thin films as a function of annealing temperature, the transmission measurements were carried out in between 300 and 2000 nm. The optical band gap values were lying in between 1.29 and 1.50 eV upon annealing the thin films in the temperature range of 300-500 degrees C. The refractive indices of the samples were in the range of 2.7-3.8 depending on the wavelength region and annealing temperature by applying the Envelope Method. The other optical constants of the samples were also calculated using Cauchy Method and Single Oscillator Model. (C) 2015 Elsevier GmbH. All rights reserved.