In　order　to　study　the　selective　inhibition　effect　of　free　nitrous　acid　(FNA)　(0.27　mg　HNO-2-N·L-1)　on　ammonium　oxidizing　bacteria　(AOB)　and　nitrite　oxidizing　bacteria　(NOB)　under　anoxic　condition,　both　the　variations　of　ammonium　oxidation　rate　and　nitrite　oxidation　rate　and　recovery　of　AOB　reactivity　and　NOB　reactivity　were　investigated　by　using　the　sludge　treated　by　FNA　for　6　h　under　anoxic　condition.　AOB　and　NOB　reactivities　decreased　by　83.57%　and　22.34%　respectively　after　the　sludge　was　subjected　to　treatment　with　FNA.　NOB　reactivity　did　not　recover　when　the　FNA-treated　sludge　was　used　under　normal　operation　environment　for　34　cycles.　Especially,　nitrite　accumulation　ratio　increased　gradually　and　remained　over　90%.　Moreover,　concentrations　of　nitrogen　compounds　in　the　typical　cycle　showed　that　nitrite　accumulation　was　not　destroyed　even　if　excessive　aeration　lasted　for　2　h.　Thus,　the　selective　inhibition　effect　of　FNA　under　anoxic　condition　could　be　used　to　establish　stable　nitritation　in　sewage　treatment　system,　providing　the　foundation　for　achieving　nitrogen　removal　from　sewage　via　nitritation　and　Anammox.　©All　Rights　Reserved