Unsupported Fe-oxide nanoparticles were used as catalysts for reverse water gas shift (RWGS) reaction at 600°C, which showed a high catalytic activity and stability. Using transmission electron microscopy, nanoparticles of Fe-oxide was found to be resistant toward agglomeration during the RWGS reaction. X-ray photoelectron spectroscopy and X-ray diffraction studies revealed that C and O formed by the reaction between Fe-oxide surface and reagent and product (CO and CO2) of the RWGS reaction diffused into the bulk of Fe-oxide nanocatalysts. As a consequence, structure of catalytically active surface, consisting of metallic Fe, was maintained during the RWGS reaction, resulting in a long-term stability of catalytic activity.