Simultaneous　removal　of　organic　and　nitrogen　from　landfill　leachate　with　high　level　of　nitrogen　by　using　single-stage　UASB-A/O　system　was　investigated.　On　the　basis　of　these,　the　feasibility　of　partial　nitrification　in　A/O　reactor　through　the　inhibition　of　FA　and　FNA　and　the　influence　factors　of　partial　nitrification　were　further　studied　in　this　work.　The　result　showed　that　denitrification　occurred　in　UASB　reactor　where　COD　and　nitrogen　were　removed　simultaneously　and　efficiently　from　landfill　leachate　with　the　removal　efficiencies　of　above　90%.　Partial　nitrification　in　A/O　reactor　was　successfully　achieved　and　maintained　through　the　simultaneous　inhibition　by　free　ammonia　and　free　nitrous　acid　at　ambient　temperature.　The　nitrite　accumulation　ratio　reached　89%-99%.　Inhibitory　mechanism　of　FA　and　FNA　was　summarized　and　described　according　to　many　reported　literatures.　It　may　be　due　to　a　direct　inhibitory　effect　of　FA　on　the　nitrite　oxidoreductase,　or　an　enzyme　involved　in　the　electron　transport　or　proton　translocation.　FNA　in　liquid　phase　diffused　into　cell　membrane　of　microbial　bacteria,　resulting　in　the　change　of　pH　in　the　cell　membrane,　further　affecting　the　proton　force　to　meet　the　requirement　of　synthesizing　ATP　via　oxidative　phosphorylation.