Treatment　of　real　leachate　from　municipal　landfill　with　high　ammonia　nitrogen　content　was　investigated　by　using　lab-scale　anoxic/anaerobic　UASB-A/O　process.　On　the　basis　of　achieving　simultaneous　COD　and　nitrogen　removal,　how　to　achieve　and　stabilize　partial　nitrification　in　the　A/O　reactor　was　studied.　Denitrification　and　methanogenesis　were　conducted　in　UASB　reactor,　and　the　average　removal　rate　of　organics　and　NOx--N　were　5.3,　1.1　kg/(m3·d),　respectively.　Partial　nitrification　was　achieved　(nitrite　accumulation　ratio　was　above　50%)　after　54　days　operation,　and　after　70days,　nitrite　accumulation　ratio　in　A/O　reactor　reached　above　90%　at　ambient　temperature　of　12　to　30.6°C.　Stable　and　long　term　partial　nitrification　was　achieved　due　to　the　cooperative　effect　of　free　acid　(FA)　and　free　nitrous　acid　(FAN).　Destruction　of　partial　nitrification　caused　by　extended　aeration　was　avoided　by　using　pH　as　process　control　parameter.　Nitrifying　population　was　optimized　by　establishing　favorable　environment　of　AOB　and　inhibiting　the　growth　of　NOB,　then　NOB　was　washed　out　from　the　biological　system,　which　was　proved　by　fluorescence　in　situ　hybridization　(FISH).