The regularities of phase formation and thermal stability in Fe3Si(111)/Si(111) structure at stepped vacuum annealing (350, 450 and 550 °C) were investigated. The layer of 32 nm Fe3Si was deposited onto Si(111) substrate by molecular beam epitaxy at 260 °C. Transmission electron microscopy (TEM) measurements demonstrated that the film thickness increases by ∼19 % at 350 °C annealing without changing the phase composition. The polycrystalline -FeSi sublayer was formed on the interface at 450 °C annealing. Further annealing at 550 °C led to the ∼ 80 nm polycrystalline film formation containing the crystallites of -FeSi, Fe5Si3, and β-FeSi2 phases.