This　study　investigates　the　effects　of　varying　COD/N　ratios　on　single-stage　partial　nitritation/anammox　(SPN/A)　process　in　a　SBR.　The　operational　period　was　divided　into　three　phases　with　different　influent　COD/N　ratios　(0.4,　0　and　0.5).　Stable　nitrogen　removal　was　achieved　in　phase　I　with　a　COD/N　of　0.4.　In　phase　II　COD　was　absent,　effluent　nitrite　and　nitrate　increased　and　nitrogen　removal　performance　gradually　deteriorated.　In　phase　III　SPN/A　failed　to　recover　from　nitrate　accumulation　when　COD/N　was　increased.　Microbial　activity　was　measured　and　microbial　community　was　analyzed　by　high-throughput　sequencing.　These　results　revealed　that　ordinary　heterotrophic　organisms　(OHO)　was　suppressed　when　influent　COD　was　absent,　leading　to　the　promotion　of　nitrification　even　at　a　low　DO　(0.2　mgL(-1)).　Therefore,　nitrite　oxidizing　bacteria　(NOB)　was　gradually　enriched　and　anammox　bacteria　was　suppressed.　Besides,　it　was　observed　that　flocs　were　sensitive　to　influent　COD　variations　than　granules,　which　requires　further　investigation.