Complete removal of CO by methanation in H-2-rich gas stream was performed over different metal catalysts. Ni/ZrO2 and Ru/TiO2 were the most effective catalysts for complete removal of CO through the methanation. These catalysts can decrease a concentration of CO from 0.5% to 20 ppm in the gases formed by the steam reforming of methane with a significantly low conversion of CO2 into methane. Catalytic activities of supported Ni and Ru strongly depended on the type of supports, i.e. ZrO2 for Ni and TiO2 for Ru are suitable supports for the methanation of CO. The effect of catalytic supports on methanation of CO could be explained by particles sizes of Ni and Ru metal. Catalytic activity of supported Ru catalysts for the complete removal of CO through methanation became higher as particle sizes of Ru metal became smaller, while Ni metal particles with relatively larger diameters were effective for the reaction. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.