Ag33Sb31Se36, thin films were prepared by thermal evaporation technique on glass substrates. X-ray diffraction analysis fur the as-deposited films showed that they have amorphous structure. On annealing at 200 degreesC films have a polycrystalline structure. The optical constants n and k of the as-deposited and annealed films have been calculated from optical transmittance and reflectance data in the wavelength range 400-1500 nm using Murmann's equations. Both n and k are practically independent on the film thickness in the range 182-478 nm. Analysis of the refractive index n yields a low frequency dielectric constant, The optical transitions are found to be allowed indirect for the as-deposited and annealed films, and the corresponding energy gaps increase with increasing annealing temperature.