Until　recently,　all　known　nitrite　oxidation　occurred　in　oxygen-rich　conditions　but　now　the　oxidation　of　nitrite　into　nitrate　within　a　low　oxygen　or　anoxic　environment　has　been　observed　in　the　ocean.　However,　this　phenomenon　is　rarely　reported　in　wastewater　treatments　and　its　mechanism　is　unknown.　In　this　study,　the　partial　nitrification　and　nitrite　oxidation　were　conducted　in　no　enough　oxygen　in　order　to　remove　nitrogen　from　landfill　leachate,　save　energy,　and　save　money.　The　results　show　that　the　NH4+-N　removal　efficiency　was　99.4%.　During　phase　I　of　the　anaerobic　sequential　batch　reactor　(ASBR),　no　change　in　Chemical　Oxygen　Demand　(COD)　and　ammonium　were　detected.　The　nitrite　concentration　decreased　from　107　+/-　3　mg/L　to　0.16　mg/L　during　96　h　of　oxygen-deficiency,　while　NO3--N　increased　from　152.5　+/-　3　mg/L　to　253.65　+/-　3　mg/L.　The　main　microorganisms　involved　in　this　reaction　in　the　ASBR　were　Nitrite-Oxidizing　Bacteria　(NOB),　including　Nitrospira　and　Nitrolancea,　their　relative　abundances　were　3.56%　and　0.13%,　respectively.　The　major　NOB　(Nitrospira)　were　confirmed　by　the　further　metagenomic　binning　analysis.　This　finding　shows　that　nitrite　oxidation　can　occur　in　oxygen-deficient　conditions　with　specific　NOB.