Iron-oxide layers formed on the surfaces of acicular iron particles and of magnetite are examined by means of Mössbauer spectroscopy. Our results indicate that thin iron-oxide layers formed on iron below 393 K consist of gamma-Fe₂O₃, where as Fe₃O₄ is formed on the surface of metallic iron if iron with thick iron-oxide layer is heated at 573 K in vacuum. Thickness of 2.1 and 3.5 nm of gamma-Fe₂O₃ is necessary to keep iron and Fe₃O₄ from air oxidation at room temperature, respectively.