Abstract: 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.