CO removal from a reformed gas by catalytic methanation on Ru/Al2O3 was studied for Polymer Electrolyte Fuel Cell (PEFC) systems. The effects of the gas composition and the reaction temperature on CO methanation activity were examined. The ratio of methanation of CO to that of CO + CO2 was also investigated. Consequently, it was found that CO could be removed from a reformed gas to below 10 ppm by controlling CO. methanation by multi-stage CO methanation, while CO could not be reduced to below 10 ppm by single-stage CO methanation due to the presence of CO2.