Ultrasonic processing is an effective tool to attain required mixing while providing the necessary activation energy in the field of biofuels. In this regard, optimization of fast transesterification of waste cooking oil is very important. The goal of this research paper is therefore to determine the effect of important parameters such as methanol to oil molar ratio, catalyst concentration (potassium hydroxide), temperature, and horn position on oil conversion to methyl ester in ultrasonic mixing method. Result of experiments showed that the optimum conditions for the transesterification process have been obtained as molar ratio of alcohol to oil as 6: 1, catalyst concentration of 1 wt.%, temperature as 45 degrees C, and horn position at the interface of methanol to oil. The results show that the ultrasonic method decreases the reaction time as much as up to eight times compare to the conventional stirring. For practically evaluating the theoretical optimum point using genetic algorithm, the obtained values were verified experimentally. In order to perform this, the catalyst concentration, temperature, and the time of reaction were determined, and the values are 1%, 48 degrees C, and 449s, respectively. For the obtained values, the biodiesel conversionwas 93.2%, so that the experimental optimum value is closed to that of the theoretical values. As a result, experimental data confirmed the obtained values from optimization method in this research work.