In　this　study,　controlled　C/N　effects　on　fast　start-up　and　stable　performance　of　partial　nitrification　process　at　15　in　a　Sequenced　Batch　Biofilm　Reactor　(SBBR)　were　investigated.　The　results　showed　that　partial　nitrification　successfully　fast　initiated　when　C/N　was　1.5　but　failed　when　C/N　was　0/3　during　60　cycles.　Fluorescence　in　situ　hybridization　and　confocal　laser　scanning　microscope　(FISH-CLSM)　results　showed　that　ammonia　oxidizing　bacteria　(AOB)　was　found　as　the　dominant　bacteria　population　when　C/N　was　1.5.　When　C/N　were　0/3,　there　were　almost　no　existence　of　AOB　and　nitrite　oxidative　bacteria　(NOB).　Partial　nitrification　could　be　stably　achieved　without　carbon　source.　However,　the　addition　of　an　appropriate　amount　of　carbon　can　effectively　improve　the　nitrification　performance,　and　it　is　better　for　the　stable　operation　of　partial　nitrification.　In　this　experiment,　partial　nitrification　was　successfully　initiated　at　high　dissolved　oxygen　(DO)　(about　9　mg•L-1)　conditions.　The　average　DO　was　maintained　at　about　6.5　mg•L-1　during　the　stable　operation,　which　successfully　decoupled　partial　nitrification　from　low　DO　concentration.　Excessive　residual　ammonium　concentration　in　the　reactors　effectively　repressed　the　growth　of　NOB　and　guaranteed　the　stable　operation　of　partial　nitrification.　At　15,　full　nitritation　was　more　suitable　for　sidestream　wastewater,　while　mainstream　wastewater　was　more　suitable　for　partial　nitritation.　©　2019,　Science　Press.　All　right　reserved.