The　nitrosation　rate　was　around　85%　after　33　continuous　cycles　(20d)　with　the　alternative　influent　of　high　and　low　ammonium　wastewater　in　a　sequencing　batch　reactor　(SBR)　under　low　oxygen　(0.15　similar　to　0.40mg/L).　The　growth　of　nitrobacteria　was　inhibited　successfully　which　led　to　steady　and　high　accumulation　of　nitrite.　Factors　of　accumulation　of　nitrite　were　investigated　and　the　effects　of　DO　and　pH　were　analyzed　during　the　startup　for　nitrosation.　The　results　showed　that　it　could　improve　the　efficiency　of　nitrosation　when　DO　concentration　was　increased　appropriately.　The　activity　of　nitrite　oxidizing　bacteria　(NOB)　was　recovered　gradually　when　DO　was　higher　than　0.72mg/L.　The　key　factor　of　nitrosation　reaction　was　the　concentration　of　free　ammonia　(FA),　while　the　final　factor　was　the　concentration　of　DO.　pH　was　a　desired　controlling　parameter　showing　the　end　of　nitrification　in　a　SBR　cycle,　while　DO　concentration　did　not　indicate　the　end　of　SBR　nitrification　accurately　because　it　increased　rapidly　before　ammonia　nitrogen　was　oxidized　absolutely.