We found that hydrogenated amorphous Si/Si(001) thin films of thickness less than about 1000 Angstrom crystallize into epitaxial silicon during solid phase crystallization. The epitaxial solidification is in contrast to the formation of polycrystalline silicon of thicker amorphous films. The structural properties of the epitaxial Si films were characterized by synchrotron x-ray scattering and transmission electron microscopy. The crystalline order in vertical direction is coherent throughout the film thickness, and the surface remains smooth. There were significant structural defects localized at the interface that distinguish the epilayer from the substrate. (C) 2004 American Vacuum Society.