The axial orientation of molecular-beam-epitaxy (MBE)-grown Fe3Si(111)/Ge(111) hybrid structures was investigated by Rutherford backscattering spectroscopy. We confirmed that during MBE above 300 °C, the interdiffusion of Fe and Ge atoms results in a composition change and the epitaxial growth of FeGe in Fe3Si. Low-temperature (≪200 °C) MBE can realize fully ordered DO3–Fe3Si with highly axial orientation [minimum yield (χmin)=2.2%]. Postannealing above 400 °C results in a composition change and the degradation of axial orientation in the off-stoichiometric Fe3Si. The significance of stoichiometry with regard to thermal stability and the interfacial quality of Fe3Si(111)/Ge(111) hybrid structures was also discussed.