Anaerobic digestion of sludge is considered to be one of the most appropriate technologies for energy production via conversion of organic matter into methane containing biogas. Biogas produced by anaerobic digestion of municipal wastewater sludge can be utilized as fuel to offset heat and electricity consumption of wastewater treatment facilities. Modeling can be used effectively to evaluate the impact of different process and operation variables on the performance of the treatment processes including complex ones such as anaerobic digestion. Therefore, the Anaerobic Digestion Model No. 1 (ADM1) was applied in order to evaluate the performance of a full-scale anaerobic sludge digester in this study. With the calibration of the most sensitive parameters used in the ADM1, model outputs corresponded well with the measured data obtained from the operation of the full-scale digester. The model was validated with the data acquired from the same anaerobic digester at a different operation period (360days) and simulation results successfully predicted the digester performance. The overall results of this study demonstrated that the ADM1 can be used as a very useful tool to simulate the digestion of mixed sludge generated from full-scale wastewater treatment plants under mesophilic conditions.