This study aimed to investigate the conversion of the pre‐treated oil, which was originally extracted from grease trap waste (GTW), to biodiesel via a transesterification reaction. The pre‐treated oil, which consists of approximately 80% free fatty acid ethyl esters (FAEEs) and 18% of glycerides, was obtained through the esterification of GTW reported in a previous study. Response Surface Methodology (RSM) based on Central Composite Design (CCD) was applied to determine the optimum conditions for the transesterification reaction through four process variables, these being: the molar ratio of ethanol to oil, reaction time, reaction temperature, and base catalyst loading. A set of 30 experiments were conducted in duplicate to study the interactive effects of the above parameters on the ester yield. A maximum ester yield of 96.7% was obtained at the optimum conditions, which were: 4:1 ethanol to oil molar ratio, 1 hr reaction time, 65 °C reaction temperature, and 1 wt% KOH loading per oil weight. In order to reduce the excess ethanol used, a modification was applied to the experimental design of the transesterification reaction, resulting in a 30% reduction in ethanol used, while the same ester yield could be obtained.