Effect of partial metal reduction on the property of supported iron catalysts has been studied using 9.1wt% Fe/SiO₂ as a model catalyst and CO hydrogenation as a test reaction. The extent of iron reduction does not alter the intrinsic activity of iron nor distribution of the hydrocarbon products with specific carbon numbers, but significantly changes the olefin/paraffin ratio among the products. This kinetic behavior agrees with the result of gas chemisorption that the H₂/CO adsorption ratio decreases as the catalyst is reduced poorly. Well-reduced iron catalysts deactivate during the reaction because the carbon species deposited on the surface gradually transform into less-reactive ones as observed in the TPSR experiments.