The effect of composition on the optical constants of Se 100xTex (x = 30, 50, 70) chalcogenide thin films of thickn ess 150 nm are studied. The film is deposited on glass substrate using thermal evaporation technique under vacuum. X-ray diffraction reveals that the poorly Te content Se 70Te30 film has amorphous structure while the other films have crystalline phases. Both the transmittance and reflectance are strongl y dependent on the film composition. The analyses of optical measurements show that the Se 30- Te70 film has two absorption mechanisms which are direct and indir ect allowed transitions giving raise two optical energy gaps. Increasing of Se content changes the direct gap to an indirect one. The increase of Se contents (from x = 30 to 70) leads to increase in the indirect gap from 1.33 to 1.85 eV. The absorption coefficient, refractive index, extinction coefficient and dispersion energy of the films are found to be depende d on the film composition.