Cu3In17Se80 thin films were prepared by thermal evaporation technique on glass and quartz substrates. X-ray diffraction analysis for the as-deposited and annealed thin films at 423K and 473 K showed that they have amorphous structure nature. The optical constants (refractive index n and absorption index k) of the as-deposited and annealed films for several samples of different thickness (from 11 nm to 61.4 nm) have been calculated from optical transmittance and reflectance data in the wavelength range 400-2500 nm. Analysis of the refractive index, n, yields high frequency dielectric constant of 8.24 for the as-deposited films which increases with annealing. The optical transitions from absorption coefficient (α) analysis are found to be allowed indirect for the as-deposited and annealed films, and the corresponding energy gap is 1.1 eV for the as-deposited films, which increases with increasing annealing temperature.