Reflection High Energy Electron Diffraction (RHEED) intensity oscillations during the growth of alternating Ge and Si layers were observed, and the growth behavior during molecular beam epitaxy of Ge/Si ultra-thin multi-layered structures on Si(l00) and Si(lll) substrates has been studied. By using RHEED intensity oscillation, we have fabricated the Ge(2ML)/Si(2ML) and Ge(4ML)/Si(4ML) ultra-thin superlattices by phase-locking method.