The role of MgO powder in preventing defluidization during the fluidized bed reduction of fine iron ores with different iron valences is investigated. The inhibiting effect of MgO powder on different types of fine iron ores is the same at 973 K, whereas it is sequenced as FeO> Fe3O4> Fe2O3 at 1073 and 1173 K. Specific diffusion couples are employed to clarify the valence-dependent inhibiting mechanism. Electron probe microanalysis, X-ray diffraction results, and the estimation of diffusion activation energy indicate that the inhibiting effect of MgO powder on defluidization is mainly caused by the physical barrier effect for all iron oxides below 1073 K, while the defluidization prevention of Fe3O4 and FeO at 1173 K is mainly attributed to the formation of Fe2MgO4 and MgO . FeO compounds on the surface, respectively. As a result, the required MgO content to prevent defluidization is sequenced as m(Fe2O3)> m(Fe3O4)> m(FeO) at 1173 K. Consequently, the MgO addition at the FeO stage will be more effective in preventing defluidization than the addition at Fe2O3 and Fe3O4 stages during multi- stage fluidized bed reduction of fine iron ores.