We have developed a facile synthesis route to 1 D structures of ultrathin polycrystalline hematite and goethite-hematite core-shells. One-dimensional structures of pure hematite and the goethite-hematite core-shell with very small diameters were synthesized in aqueous solution at low temperature with a simple, rapid method based on the oxidation of Fe3O4 nanoparticles without using surfactants.