The effects of magnetic fields on the transformation of rust (hematite) to magnetite (passivity state) were investigated under a fluid flow condition. The composition variation of rust was analyzed every 15 days by X-ray diffraction analysis to investigate the transformation mechanism. Samples used were rusted iron plates and fine powders of hematite and magnetite, which are chemical reagents. The magnetic flux density of the magnetic treatment device was 300mT. Distilled water was used as fluid and the fluid flow velocity was 2 m/s. The physical and chemical properties were changed with long duration. Temperature and pH of fluid increased to 20°C and 2, respectively in comparison with initial values. On the other hand, DO decreased 2 mg/l from the initial value. Rust was transformed to mixtures of magnetite and FeOOH after 30∼45 days and transformed to magnetite completely after 90 days. No transformation of composition was observed to hematite and magnetite in aqueous slurries of fine powders of hematite and magnetite. In the transformation of rust to magnetite in the magnetic field, the following reactions are very important; Fe2++8FeOOH+2e-→3Fe3O4+4H2O, Fe2O3+6H++2e-→2Fe2++3H2O, Fe+2Fe3+→3Fe2+. Lorentz force effects will cause the generation of electrons in these reactions and boost the transformation reactions from an electrochemical point of view.