The formation of bismuth orthoferrite under hydrothermal conditions at temperature 160, 180, or 200°С and pressure 100 MPa in aqueous solution of potassium hydroxide has been studied. The determined composition and structure of polycrystalline phase with sillenite structure have evidenced its formation at the interface of the crystallites of amorphous iron oxide. It has been shown that the formation of polycrystalline round-shaped BiFeO3 particles with size about 20 μm occurs via aggregation of perovskite-type phase crystallites (38–70 nm). Pycnometric density of BiFeO3 and the amorphous phase has been determined, and Mossbauer spectra reflecting the state of iron in the phases coexisting during the formation of BiFeO3 have been analyzed.