Partial　nitritation　is　the　basis　of　anoxic　ammonium　oxidation　(Anammox),　however,　very　limited　research　has　been　undertaken　on　this　field.　In　this　study,　an　A/O　reactor　was　used　to　investigate　the　feasibility　of　the　partial　nitritation　and　the　performance　of　biological　phosphorus　removal　from　municipal　wastewater.　The　obtained　results　showed　that　the　partial　nitritation　could　be　achieved　in　A/O　reactor　with　nitrite　accumulation　rate　of　85%.　It　was　possible　that　nitrite-to-ammonium　ratio　in　the　effluent　from　the　A/O　reactor　was　approximately　controlled　at　1.0　by　adjusting　hydraulic　retention　time,　which　was　beneficial　to　the　Anammox　reaction.　The　fluctuation　of　temperature　and　DO　concentration　might　lead　to　the　deterioration　of　nitritation.　After　the　achievement　of　partial　nitritation,　the　biological　phosphorus　removal　was　not　stable,　which　might　relate　to　the　increase　of　FNA　concentration　in　the　effluent　of　A/O　reactor,　and　needed　further　experimental　investigation.