The unreacted-core model has been extensively used for the analysis of the gaseous reduction rate of the iron oxide because of its simplicity and relatively a good agreement of the experimental result with the calculated reduction curve. However, the rate parameters in the model are known to be affected by many factors such as the reactivity, the diffusivity in the reactant zone and through the product layer, and the size- and shape of the solid particle, so that it is criticized that the applicability of this model is rather restricted. In this study, the applicability of the unreacted-core model was reviewed based on the intermediate model. It was found that the unreacted-core model showed a good applicability when Thiele`s modulus was higher than 20 and effective diffusivity through the product layer was larger than 10 times of that in the reactant zone. However, it showed worser applicability when the effective diffusivity through the product layer becomes closer to that in the reactant zone even in case of a higher Thiele`s modulus than 20.