By using compositionally graded SiGe films as virtual substrates, tensile strained Si films with the strain of 1.5% and the threading dislocation density less than 1.0×105cm-2 are successfully grown in micron size windows by molecular beam epitaxy (MBE). The thickness of the virtual substrates was only 330nm. On the surface of the s-Si films no cross-hatched lines resulting from misfit dislocations could be observed. We attribute these results to the edge-induced strain relaxation of the epitaxial films in windows, and the patteed virtual substrates with compositionally graded SiGe films.