Fe layer with thickness of 10 Angstrom was deposited on Si (111) 7 x 7 surface by molecular beam epitaxy (MBE) in an integrated ultrahigh vacuum (UHV) system. After annealing at 873 K, epitaxial beta-FeSi2 islands with single direction moire fringes were grown on Si substrate. In situ transmission electron microscopy (TEM) observation reveals that there is an epitaxial relationship between beta-FeSi2 and substrate, and epitaxial beta-FeSi2 islands are in strain state different from bulk beta-FeSi2 crystal. The beta-FeSi2/Si interface structure was characterized by in situ plan-viewed and ex situ cross-sectional high-resolution TEM (HRTEM) analyses. It is clear that the misfit dislocations exist at the beta-FeSi2/Si interface to accommodate the lattice parameters' mismatch. A model describing the beta-FeSi2/Si interface configuration has been established. (C) 2003 Elsevier Science B.V. All rights reserved.