Amorphous Ga{sub 20}S{sub 75}Sb{sub 5} and Ga{sub 20}S{sub 40}Sb{sub 40} thin films were prepared onto glass substrates by using thermal evaporation method. The effect of annealing (under vacuum) at different temperatures on the optical parameters was investigated in the temperature range 373-593 K. The optical absorption coefficient ({alpha}) for the as-deposited and annealed films were calculated from the reflectance and transmittance measurements in the range 190-900 nm. X-Ray diffraction indicates that the as-deposited films and those annealed up to the glass transition temperature (T{sub g}) exhibit amorphous state. On annealing above the glass transition temperature these films show a polycrystalline structure. Analysis of the optical absorption data indicates that the optical band gap E{sub g}{sup opt} of these films obeys Tauc's relation for the allowed non-direct transition. It was found that the optical band gap E{sub g}{sup opt} increases with annealing temperature up to T{sub g}, whereas above T{sub g} there is a remarkable decrease. The obtained results were interpreted on the basis of amorphous- crystalline transformation.