A　lab-scale　two-stage　UASB-A/O　combined　process　for　treating　the　real　municipal　waste　leachate　and　a　pilot-scale　SBR　for　treating　the　municipal　wastewater　were　used　to　investigate　the　inhibition　kinetics　of　FA(free　ammonia,　FA)　and　MLSS(Mixed　liquor　suspended　solids,　MLSS)　to　nitrification　rate.　Under　the　premise　of　achieving　short-cut　nitrificationdenitrification　stably　(running　130　days)　in　UASB-A/O　and　favorable　nitrogen　and　phosphorus　removal　efficiency　in　SBR,　a　number　of　nitrification　batch　test　was　carried　out　in　different　concentrations　of　FA　and　under　different　MLSS　gradient　with　sludge　which　was　from　A/O　reactor　and　SBR　reactor.　A　nitrification　inhibition　kinetics　model　was　established　based　on　a　large　number　of　experiments　data　and　the　FA　nitrification　inhibition　kinetic　constants　under　different　MLSS　were　deduced　by　function　match.　The　relation　curves　of　FA　concentration　and　specific　rate　of　nitrification　match　the　Andrews　inhibition　kinetics　model　well　at　constant　temperature　and　MLSS　conditions.　The　both　maximum　specific　nitrification　rate　(rmax)　and　half-saturation　constant　(KFA)　were　maximum　at　MLSS=2000mg/L,　but　they　vary　greatly　at　different　MLSS　conditions.　That　the　rmax　and　KFA　of　A/O　were　33.966　mgNH4+-N/g　MLSS·h　and　3.297　mg/L　under　MLSS=2000mg/L,　respectively,　was　higher　than　the　values　of　SBR.　Both　the　inhibition　constant　(KI)　were　increased　with　the　increase　of　MLSS,　however,　the　A/O　sludge＇s　increased　significantly　(form　4.980mg/L　to　39.276mg/L)　which　means　the　inhibition　was　weaker　when　MLSS　was　high　especially　for　the　adapted　sludge.　©　2011　IEEE.