A reduction reaction model. of fine iron ore in a circulating fluidized bed, which involved inner and outer circulation of powder, was developed to obtain scale-up information and to examine the effects of operating factors on reduction degree. Differential equations of mass balance were solved numerically under a one-interface unreacted core model by the Runge-Kutta-Gill method. Calculated curves of reduction degree vs. reduction temperature, consumed gas volume per supplied iron ore, and mean residence time agreed with experimental ones. Reduction degree decreased with increasing input gas oxidation degree. However, the differential changed discontinuously depending on whether reduced iron oxides or metallic iron were in equilibrium with the gas composition in the circulating fluidized bed. This model enabled us to predict gas oxidation degree according to the targeted reduction degree of iron ore.