The　nitrosation　rate　was　around　85%　after　33　continuous　cycles(20　d)　with　the　alternative　influent　of　high　and　low　ammonium　wastewater　in　a　sequencing　batch　reactor(SBR)　which　low　oxygen(0.15-0.40　mg/L).　The　growth　of　nitrobacteria　was　inhibited　successfully　which　led　to　a　steady　and　high　accumulation　of　nitrite.　The　effects　of　five　different　DO　concentrations　were　investigated　with　low　ammonium　influent　on　nitrite　accumulation.　The　results　clearly　showed　that　ratios　of　the　nitrite　accumulation　rate　[Δc(NO2--N)/t]　to　that　of　nitrate　accumulation　[Δc(NO3--N)/t]　were　16.37,　17.28,　17.69,　7.29,　3.64　at　five　different　DO　concentrations(0.27,　0.48,　0.72,　0.99,　1.47　mg/L　),　respectively.　The　corresponding　nitrosation　rates　were　95.28%,　94.49%,　94.64%,　87.84%　and　78.63%　at　the　temperature　of　22.3-27.6°C.　Results　showed　that　a　maximum　of　94%　nitrosation　efficiency　was　reached　with　average　DO　concentration　of　0.72　mg/L　and　the　aeration　time　of　4　hours　at　normal　temperature　after　23　continuous　cycles(13　d).