BACKGROUND AND OBJECTIVE: Leachate, which contains large amounts of ammonium and hazardous organic compounds, can lead to the pollution of surface water and groundwater consequently, leachate collection and treatment are essential before discharge into the environment. Anaerobic digestion is one of the most cost-effective methods of contaminated wastewater treatment. In this study, we aimed to evaluate the efficiency of an anaerobic digester in simultaneous removal of chemical oxygen demand (COD) and ammonium from landfill leachate. METHODS: In this experimental study, a cylindrical anaerobic digester, made of Plexiglas sheets (with an inner diameter of 240 mm and useful volume of 10 L), was loaded with landfill leachate in Ghaemshahr, Iran. The effects of temperature (ambient temperature, 35°C, and 55°C) and various hydraulic retention times (1-5 days) on anaerobic digestion efficiency in COD and ammonium removal were assessed. FINDINGS: At a hydraulic retention time of five days, maximum COD and ammonium removal (94% and 36%, respectively) was reported at thermophilic and ambient temperatures, respectively. The increase in hydraulic retention time had a positive impact on the efficiency of the digester in removing organic compounds and ammonium. Moreover, the rise in anaerobic digester temperature improved COD and ammonium removal. CONCLUSION: According to our findings, the developed anaerobic digester could be used as a convenient and efficient tool for removing organic matters from landfill leachate. However, given the low efficiency of this digester in ammonium removal, an additional aerobic stage is required for wastewater treatment.