In　this　study,　the　nitrogen　removal　performance　and　microbial　characteristics　of　completely　autotrophic　nitrogen　removal　over　nitrite　(CANON)　process　was　investigated　with　a　step-wise　increasing　of　C/N　ratio　(0.5,　1,　2　and　4)　in　a　membrane　bioreactor.　The　microbial　distribution　of　aerobic　ammonia-oxidizing　bacteria　(AOB)　and　anaerobic　AOB　(AAOB)　was　analysed　by　fluorescence　in　situ　hybridization　(FISH).　Results　showed　that　the　denitrification　ratio　rose　up　correspondingly　with　the　increase　of　influent　C/N,　and　nitrogen　removal　rate　(NRR)　reached　the　maximum　when　C/N　was　1　due　to　the　harmonious　work　of　denitrification　and　CANON.　However,　NRR　decreased　when　influent　C/N　was　more　than　2.　The　threshold　C/N　ratio　of　CANON　process　was　2.2;　so　the　sewage　with　a　high　C/N　ratio　should　be　pretreated　by　combining　with　pre-oxidation　of　organics　or　anaerobic-energy-producing　process.　FISH　results　showed　decreasing　numbers　of　both　AOB　and　AAOB　with　the　addition　of　organics.