Abstract: The reactivity and structural characterizations of the sintered iron ore in redox cycles were investigated. The sintered return fines were selected as oxygen carriers and the chemical looping combustion was tested in a fluidized bed with a reducing atmosphere of 95% methane. The characterizations of structural and physical properties of the sintered raw materials and the reduced and oxidative regenerated samples were conducted. The results reveal that during the initial oxidation cycle, the oxygen carrying capacity and reaction activity of the sinter are improved remarkably. The specific surface area of the sinter increases significantly in the first 25 cycles, which may be one of the main reasons for the reactivity increase of the sinter. During the recycling process, some cracks are formed on the surface of sintered return fines and gradually get development. The Raman results indicate that the new crystalline phase, lepidocrocite (γ-FeOOH), is formed, which will decrease the oxygen carrying capacity of the oxygen carrier. When the methane is used as the reducing gas, there is no carbon deposition on the surface.