An oxygen-deficient magnetite (Fe3O4–8) has been found to form by passing H2 gas through magnetite powder at 300°C with its spinel structure retained. The oxygen-deficient magnetite is a metastable phase in the transformation of magnetite into α-Fe. The lattice constant of the oxygen-deficient magnetite enlarged to 0.8407nm in the maximum value which is substantially larger than that of the stoichiometric magnetite (a0=0.8396nm). The formation mechanism of the oxygen-deficient magnetite was studied at 300°C using two specimens of magnetite powders with developed orientations on the (111) and (100) planes. These are referred to as (111)- and (100)-magnetite, respectively. The formation of α-Fe was suppressed over the crystal of (111)-magnetite, where wUstite and oxygen-deficient magnetite were formed while keeping the f.c.c. arrangement of the oxygen ions in the solid. On the other hand, over the (100)-magnetite, the formation of a-Fe was enhanced. It is considered that, on the (111)- magnetite, the Fe2+ ion formed by H2-reduction on the surface can move into interstices of the f.c.c. lattice of bulk to form the oxygen deficient state. This will come from the fact that the Fe2+ ion on the surface is so strongly supported with three Fe-O bondings that the Fe2+ ions cannot readily diffuse on the surface when the surface oxygen ions are removed on the (111)- magnetite.