Food waste has been regarded as the main source of various environmental pollution in Korea due to the high volatile solids (VS) and moisture content caused by the features of dietary habits. The feasibility of food waste as a co-substrate in anaerobic digestion of sewage sludge was investigated in mesophilic and thermophilic conditions using batch tests. Cumulative methane production, dissolved organic carbon (DOC) and volatile fatty acids (VFA) were monitored to find the optimal mixing ratios of food waste and sewage sludge for the enhanced performance of co-digestion. It was observed that adequately mixed food waste led to the enhanced methane production both at mesophilic and thermophilic conditions. However, a conventional linear regression conducted for the optimisation of co-substrate mixing ratios was not accurate in describing exact methane production trends of co-digestion because of the different biodegradability of substrates. Therefore, a remodified Gompertz equation showing nonlinear relationship between variables was developed to find exact information with the same experimental data obtained at 2g VS/l generally used in biochemical methane potential (BMP) tests. Based on an influential parameter, methane production rate (MPR), the optimal mixing ratios of food waste were 39.3% and 50.1% in mesophilic and thermophilic conditions, respectively. To confirm the application of the remodified Gompertz equation, secondary batch tests were conducted with the substrate concentrations of 1-4g VS/l. In overall range tested, the confident mixing ratios of food waste was adjusted to 30-40% and 40% in mesophilic and thermophilic conditions, respectively. The most significant factor for enhanced performance was the improved organic carbon content provided by additional food waste.