In this study, the effect of thermal pre-treatment (TP) on the physical characteristics and co-digestion of a mixture of food waste and sludge was investigated. The food waste (FW) to sewage sludge (SS) ratio used in this research is 1:2 (VS-based) to form a final concentration of 11.20%. The inoculum to substrate ratio was set at 1:1 (Volume-based). Undoubtedly, the results show that TP has changed the physical characteristics of the food waste to sewage sludge mixture. The results show that the pretreatment increased the biogas production from 4385 ml for the untreated reactor (RB) to 5685 for the reactor R2(140) at 140°C and the improvement in biogas production reaches 29.65% in the reactor R2(140) and the removal of volatile solids was 58.90%. Therefore, after the biomethane potential test, the temperature of 140°C was found to be optimal in the production of biogas. The optimal condition is to use a mixture of pre-treated SS at the temperature of 140°C and untreated FW, so TP is recommended to be used in anaerobic digestion of the mixture of food waste and sewage sludge.