Oxidative removal of a small quantity of CO from hydrogen atmosphere has been attempted by using catalysts consisting of 3d transition metal oxides. The oxidation of CO takes place in preference to that of H-2 on catalysts containing Co or Mn in a temperature range from 323-423 K. At higher temperatures, the oxidation of H-2 becomes to coincide and then the selectivity to the CO oxidation decreases. Hydrogenation of CO giving methane also occurs over the catalysts containing Ni or Co. Reduction of catalysts at high temperatures might be the reason for the observed change in the catalytic properties. Among the catalysts tested, CoO showed the best performance and was considered to be one of the candidates for the refinement of the hydrogen fuel for the polymer-electrolyte type fuel cells (PEFCs).