The　free　ammonia　(FA)　inhibition　on　ammonia　oxidizing　bacteria　(AOB)　and　nitrite　oxidizing　bacteria　(NOB)　under　anaerobic　condition　was　investigated　in　this　study.　The　results　indicated　that　NOB　was　more　sensitive　to　the　FA　anaerobic　treatment　than　AOB.　The　FA　anaerobic　inhibition　on　nitrifier　gradually　heightened　with　the　increase　of　FA　concentration.　Accompanied　with　FA　concentration　increase　from　0　to　16.82　mg　NH3-N　center　dot　L-1　(the　highest　concentration　adopted　in　this　study),　the　activity　of　AOB　reduced　by　15.9%,　while　NOB　decreased　by　29.2%.　After　FA　anaerobic　treatment,　nitrite　was　accumulated　during　nitrification.　However,　the　nitrite　accumulation　disappeared　on　the　sixth　cycle　of　activity　recovery　tests　with　excessive　aeration.　Based　on　this　result,　a　novel　strategy　for　achieving　nitritation　is　proposed,　which　involves　recirculating　a　portion　of　the　activated　sludge　through　a　side-line　sludge　treatment　unit,　where　the　sludge　is　subjected　to　treatment　with　FA　under　anaerobic　condition.