Performance of an anaerobic baffled reactor (ABR) used for landfill waste leachate treatment was evaluated in this study. For this purpose, the effects of different values of hydraulic retention time (HRT) and various concentrations of landfill leachate on the reactor performance during 52 d were evaluated. In this research, the system exhibited, during entire start-up period, a good performance in terms of removing chemical oxygen demand (COD), nitrate, and total Kjeldahl nitrogen (TKN). The obtained results indicated reductions in COD, TKN, nitrate, and total dissolved salts contents from 55 to 86%, from 42 to 92.4%, from 41 to 96.6%, and from 20 to 64%, respectively. During the entire start-up period, the value of oxidation–reduction potential was continuously monitored to ensure the remaining under anaerobic conditions. Also, the value of pH ranged from 6.1 to 8.2 in the course of reactor performance. Achieved at the COD concentration of 2,700 mg/l within a HRT of 48 h, maximum COD and nitrate removal performances were 86 and 96.6%, respectively. Furthermore, realized at the COD concentration of 1,800 mg/l within a HRT of 48 h, the maximum TKN removal performance was 92.4%.