A system consisting of a two-stage upfl ow anaerobic sludge blanket(UASB) reactor and an anoxic/aerobic(A/O)reactor was used to treat municipal land fill leachate. Denitri fication took place in the first stage of the UASB reactor(UASB1). The chemical oxygen demand of the UASB1 ef fl uent was further decreased in the second stage(UASB2). Nitri fication was accomplished in the A/O reactor. When diluted with tap water at a ratio of 1:1, the ammonia nitrogen concentration of the in fl uent leachate was approximately 1200 mg·L-1, whereas that of the system ef fl uent was approximately 8–11 mg·L-1, and the corresponding removal ef ficiency is about99.08%. Stable partial nitri fication was achieved in the A/O reactor with 88.61%–91.58% of the nitrite accumulation ratio, even at comparatively low temperature(16 °C). The results demonstrate that free ammonia(FA) concentrations within a suitable range exhibit a positive effect on partial nitri fication. In this experiment when FA was within the 1–30 mg·L-1range, partial nitri fication could be achieved, whereas when FA exceeded280 mg·L-1, the nitri fication process was entirely inhibited. Temperature was not the key factor leading to partial nitri fication within the 16–29 °C range. The inhibitory in fl uence of free nitrous acid(FNA) on nitri fication was also minimal when p H was greater than 8.5. Thus, FA concentration was a major factor in achieving partial nitri fication.