Since denitrification of landfill leachate was a challenging problem, this study modified SBR to improve the efficiency of nitrogen removal. The effects of several operation parameters were investigated, including operation mode, dissolved oxygen, over aeration, and carbon nitrogen ratio of the leachate. The results demonstrated that the modified SBR could reduce the COD, ammonia-nitrogen, and TKN of the leachate from about 4000 mg/L, 1000 mg/L, and 1100 mg/L to less than 500 mg/L, 5 mg/L and 40 mg/L, respectively, without extra carbon source, meaning that the removal rates could reach up to 85%, 99%, and 95%, respectively. Besides, it was found that the PHA content in denitrifying bacteria was the key factor affecting the denitrification efficiency. Nitrogen removal efficiency would be improved by high dissolved oxygen, anaerobic mixing before aeration and less over aeration. Additionally, the advanced denitrification would be achieved if the leachate carbon nitrogen ratio was more than four.