Both　＇two-stage　upflow　anaerobic　sludge　blanket　(UASB)　+　anoxic/oxic　(A/O)＇　system　and　＇one　stage　UASB　+　A/O＇　system　were　successively　introduced　to　treat　a　leachate　containing　relatively　low　concentration　of　organics　and　high　concentration　of　ammonia　from　a　mature　landfill　site.　The　＇two-stage　UASB　+　A/O＇　system　was　used　firstly.　The　denitrification　of　the　return　effluent　was　carried　out　in　the　first　stage　UASB　(UASB1).　The　nitrification　took　place　in　the　A/O　reactor.　The　results　showed　that　most　biodegradable　organic　matter　was　removed　in　the　UASB1,　so　＇one　stage　UASB+A/O＇　system　was　used　in　the　second　phase　experiment.　The　NH44-N　loading　rate　of　A/O　reactor　and　operation　temperature　were　0.28-0.60　kg　NH4+　-N·m-3·d-1　and　17-29°C,　respectively.　The　COD　removal　efficiency　and　the　final　effluent　COD　were　50%-70%,　1000-1500　mg·L-1,　respectively.　Short-cut　nitrification　with　90%-99%　of　nitrite　accumulation　efficiency　took　place　in　the　system.　The　NH4+-N　removal　efficiency　varied　between　90%　and　100%.　When　NH4+-N　loading　rate　was　less　than　0.　45　kg　NH4+/　-N·m-3·d-1,　the　NH4+-N　removal　efficiency　was　more　than　98%,　and　effluent　NH4+N　was　below　15　mg·L-1.　When　the　ratio　of　the　feed　COD　to　feed　NH4+-N　concentrations　was　2-3,　the　total　inorganic　nitrogen　(TIN)　removal　efficiency　was　70%-80%.　The　sludge　samples　from　A/O　reactor　were　analyzed　by　using　fluoresence　in　situ　hybridization　(FISH).　The　FISH　analysis　showed　that　ammonia　oxidation　bacteria　(AOB)　were　4%　of　the　eubacteria,　meanwhile　nitrite　oxidation　bacteria　(NOB)　were　less　than　0.2%　of　the　eubacteria.