A carbon-supported ruthenium catalyst with barium promoter was prepared by carbon monoxide treatment and its application as a catalyst in ammonia synthesis. CO treatment of Ru catalyst supported on carbon led to the formation of carbon and the increase of the number of anhydride, ester, and hydroxyl groups along with the decrease of the amount of quinone. Ru catalyst with CO treatment showed excellent catalytic activities and thermal stability with respect to the catalyst without treatment because the formation of carbon during CO treatment would not only help to remove the superfluous hydrogen atoms by carbon gasification but also weaken the adsorption strength of hydrogen and nitrogen.