Au/Co3O4-CeO2 mixed-oxide catalysts were shown experimentally to be highly active and selective for the oxidation of CO in hydrogen-rich mixture. Activity was markedly influenced by the composition of the support, aging temperature and Au-loading temperature. It provided that single-step removal of CO from hydrogen-rich stream both in the absence and presence of CO2 and H2O to a PEMFC tolerant level. It was found that catalytic activity is greatly affected by adding CO2 in the mixture and increased by farther adding H2O. It meants H2O has the effect to rise catalytic activity. Moreover,it shows better stability with reaction time for the preferential CO oxidation.