An　UASB+Anoxic/Oxic　(A/O)　system　was　introduced　to　treat　a　mature　landfill　leachate　with　low　carbon-to-nitrogen　ratio　and　high　ammonia　concentration.　To　make　the　best　use　of　the　biodegradable　COD　in　the　leachate,　the　denitrification　of　NOx--N　in　the　recirculation　effluent　from　the　clarifier　was　carried　out　in　the　UASB.　The　results　showed　that　most　biodegradable　organic　matters　were　removed　by　the　denitrification　in　the　UASB.　The　NH4+-N　loading　rate　(ALR)　of　A/O　reactor　and　operational　temperature　was　0.28-0.60　kg　NH4+-N/(m(3)center　dot　d)　and　17-29　degrees　C　during　experimental　period,　respectively.　The　short-cut　nitrification　with　nitrite　accumulation　efficiency　of　90%-99%　was　stabilized　during　the　whole　experiment.　The　NH4+-N　removal　efficiency　varied　between　90%　and　100%.　When　ALR　was　less　than　0.45　kg　NH4+-N/(m(3)center　dot　d),　the　NH4+-N　removal　efficiency　was　more　than　98%.　With　the　influent　NH4+-N　of　1200-1800　mg/L,　the　effluent　NH4+-N　was　less　than　15　mg/L.　The　shortcut　nitrification　and　denitrification　can　save　40%　carbon　source,　with　a　highly　efficient　denitrification　taking　place　in　the　UASB.　When　the　ratio　of　the　feed　COD　to　feed　NH4+-N　was　only　2-3,　the　total　inorganic　nitrogen　(TIN)　removal　efficiency　attained　67%-80%.　Besides,　the　sludge　samples　from　A/O　reactor　were　analyzed　using　FISH.　The　FISH　analysis　revealed　that　ammonia　oxidation　bacteria　(AOB)　accounted　for　4%　of　the　total　eubacterial　population,　whereas　nitrite　oxidation　bacteria　(NOB)　accounted　only　for　0.2%　of　the　total　eubacterial　population.