Catalytic properties of mordenite-supported iron catalysts prepared by various methods were charac-terized by means of several measurement techniques such as. CO chemisorption, oxygen titration, Mossbauer spectro-scopy(MES) and Electron spin resonance(ESR). Based on the characterization results, catalytic activity and selectivity toward olefin in the CO hydrogenation reaction were explained. Iron-loaded mordenites showed various forms of iron particles upon reduction; α-Fe₂O₃, superparamagnetic oxide, ion-exchanged Fe⁺², and Fe^o. The reduced iron came mostly from α-Fe₂O₃, and from superparamagnetic oxide in minor amount. Iron on Na-mordenite showed higher olefin selectivity than that on HM because of higher dispersion of iron particles and less hydrogen-donating acidic site on NaM. Small amount of platinum added to iron enhanced both the production and reduction of superparamagnetic oxide particles. Olefin selectivity in the CO hydrogenation increased in the order of Fe/HM<Fe/SiO₂<Fe/NaM<Pt/Fe/NaM. Vapor deposition of silicon on the external surface of Fe/NaM also increased the olefin selcetivity.