Hydrogenation of carbon monoxide was carried out at P=10atm, T=325°C, and GHSV=1, 000-5, 000h-1 over chlorine-containing magnetite catalysts in the slurry phase. High selectivities to light olefins (C=2-C=4) of 48% with suppression of methane formation to 11% were obtained with these catalysts. Carbon dioxide, a by-product of the reaction, was hardly produced over the catalysts. The chlorine content in the used catalysts, determined by X-ray fluorescence spectrometry, was in the atomic ratio range of Cl/Fe=0.002-0.01. The XPS results of the catalysts showed that the surface concentration of chlorine was in the range of Cl/Fe=0.1-0.2, and that the chlorine on the surface was more negatively charged than that of the iron chloride. The chlorine prevented over-reduction of the surface to metallic iron during the reaction. These effects were explained in terms of electron attraction of the chlorine on the surface.