Carbon-supported　Ru　catalyst　with　alkali　promoter　is　arguably　the　most　significant　advancement　in　ammonia　synthesis.　However,　the　insufficient　stability　of　this　type　of　catalysts　greatly　limits　their　applications,　and　carbon　methanation　is　often　considered　as　the　reason　for　deactivation.　Here　we　report　that　there　is　no　clear　correlation　between　carbon　methanation　and　the　loss　of　activity.　Rather,　carbon　oxidation　at　high　temperature,　especially　via　the　formation　of　carbon　monoxide,　was　found　to　have　a　strong　influence　on　the　activity　and　stability　of　Ru　catalysts.　As　a　result,　there　is　a　significant　increase　in　carbon　loss,　leading　to　the　sintering　of　Ru　particles　and　thereby　altered　hydrogen　adsorption.　Consequently,　catalytic　activity　and　stability　of　Ru/C　catalysts　are　reduced.　This　finding　contributes　to　the　development　of　strategies　to　design　carbon-supported　metal　catalysts　with　high　activity　and　stability.