In　this　study,　effect　of　C/N　ratio　on　denitrification　were　investigated　using　four　sets　of　parallel　catching　bed　reactors,　which　were　using　acrylic　resin　fiber　(ARF)　as　carrier　materials.　The　results　indicate　that　this　process　which　was　used　in　wastewater　treatment　of　civil　engineering　can　get　better　COD　and　nitrogen　removing　performance.　NH4+-N　removal　rate　reduced　with　the　increasing　of　C/N　ratio,　and　the　average　removal　rate　of　COD　and　the　total　nitrogen　(TN)　increased　when　C/N　ratio　is　increased.　When　C/N　ratio　exceeded　12,　TN　removal　rate　has　no　obvious　growing.　Meanwhile,　fluorescent　in　situ　hybridization　(FISH)　analysis　indicated　that　the　biomass　in　the　biofilm　were　much　richer　than　which　in　the　suspension,　and　the　ammonia　oxidizing　bacteria　have　a　easy　access　to　be　dominant　bacterial　community　in　lower　C/N　ratio.