Anaerobic codigestion of municipal wastewater sludge and restaurant grease was investigated in a semi-continuous lab-scale digestion experiment under mesophilic condition (37 °C). Compared to the control digester, COD loading rate for test digester was elevated to 387% (organic loading rate 4.235 kgVS/m3/d) and led to 467% increase in daily biogas production, 25.2% increase in methane yield (based on VS deduction), 29.8% increase in COD reduction rate and 27.2% increase in VS reduction rate, respectively. Methane content ranged from 62% to 67%. There was no negative effect of grease addition on the digester performance in this experiment. The great increases in biogas production and methane yield indicated enhanced digestion performance. In addition, partial alkalinity and pH proved to be good indicators to monitor digestion process and predict overloading. It is still possible to keep increasing organic loading by the addition of more grease, but according to monitoring results, the system was close to overloading.