Abstract: A series of 20%NiO(Cl_x)/CeO₂ samples with feed atomic ratio of Cl/Ce at x=0.0~0.30 were prepared by impregnation method and used to selective methanation of CO in H₂-rich gas (CO-SMET) after reduction. It is found that chlorine doped suppressed CO₂ conversion greatly. And CO concentration was thus decreased to below 10^-5 over 20% Ni(Cl_0.2)/CeO₂ catalyst with CO methanation reaction selectivity not less than 50% in a wide reaction temperature range of 220~240 ℃, meeting the requirement for fuel of proton exchange membrane fuel cell (PEMFC). In order to reveal effect of chlorine doping, measurements of X-ray diffraction (XRD), temperature programmed reduction (TPR) and adsorption isotherms of CO and CO₂ were conducted. Phase composition of the samples and crystallite size of CeO₂ were estimated by the Rietveld method. Thickness of crystallites of NiO or Ni was estimated by the Scherrer equation.