Anaerobic baffled reactor (ABR) is one of the widely used wastewater treatment systems in industrial and domestic applications. In this study, the effect of dilution rates (5%, %10, 20%, 50%) on the landfill leachate (LFL) with regard to chemical oxygen demand (COD), color, nitrogen compounds, and organic matter was investigated. The maximum removals were observed when the dilution rate was 20% (v:v,1:5). COD, dissolved organic carbon (DOC), total nitrogen (TN), color, nitrate (NO3-) and ammonium (NH4+) removal efficiency was approximately 81%, 61%, 15%, 17%, 1% and 5%, respectively. The results indicated that the adverse effects of the dilution rate on the removal of contaminants are high when it is higher than 1:5 (v:v). The study suggests that the dilution of leachate presents a significant effect on the treatment performance.