Grease trap waste (GTW) often has a very high content of free fatty acids (FFA) which requires a pre‐treatment step before it can be converted to biodiesel via the transesterification reaction. In this study, the esterification reaction was performed to reduce the level of FFA in the GTW, utilizing ethanol as the reagent in the presence of the catalyst H2SO4. Response Surface Methodology (RSM) based on Central Composite Design (CCD) was applied to determine the optimal conditions for the esterification reaction through four process variables, these being: the molar ratio of ethanol to fats, oils, and grease (FOG), reaction time, reaction temperature, and catalyst loading. A set of 30 experiments were conducted in triplicate to study the interactive effects of the above variables on the reduction of the level of FFAs. An FFA% of 0.84% was obtained at the optimum conditions, which were 6:1 ethanol to FOG molar ratio, 3 hr reaction time, 750C reaction temperature, and 3 wt% H2SO4 loading per FOG weight.