The epitaxial film of semiconductor Mg2Si was prepared on Si(100) substrate by DC magnetron sputtering system.The crystal structures and the surface morphology of the Mg2Si films were characterized by X ray diffraction (XRD) and field emission scanning electron microscope(FESEM).The epitaxial orientation property of Mg2Si film was obtained by analyzing the effect of the extinction property of Mg2Si film theoretically. It was found that the epitaxial Mg2Si film prepared on the Si(100) has only one preferred orientation of Mg2Si(220).