A novel CO conversion catalyst, Sm2CU2/3Nb4/3O7 With a pyrochlore structure, has been synthesized. Catalytic oxidation of carbon monoxide on the catalyst can occur at about 50 degrees C. The conversion efficiency will be close to 100% at near 300 degrees C. Sm2CU2/3Nb4/3O7 is an insulator. The possible effect of variation of the valence states of Sm, Cu and Nb ions in the compound on the oxidation reaction of carbon monoxide has also teen discussed.