Thin films of [(PbTe)1-x(PbSe )x] have been prepared by thermal evaporation of about 1.3x10^ -3 Pa. Film thicknesses have been determined using multiple -beam Fizeau fringes at reflection. The thickness of the films were ranged between 60 nm and 180 nm. X-ray diffraction patterns reveal the formation of polycrystalline films. Optical gap has been estimated. It was found that the gap depends on the film thickness. The obtained results are discussed in terms of the band structures of PbTe and Pbse.