This paper evaluates the production of methyl esters front soybean waste oil and methanol by the acid catalytic process. The reaction was carried out applying low-frequency high-intensity ultrasound (40 kHz) under atmospheric pressure and ambient temperature. Response surface methodology (RSM) was used to evaluate the influence of the alcohol/oil molar ratio and catalyst concentration (sulfuric acid) oil the yield of soybean waste oil into methyl esters. Analysis of the operating conditions by RSM showed that the most important operating condition affecting the reaction was the alcohol/free fatty acids (FFAs) molar ratio. The highest yield observed was of 99.9% after 60 min of reaction. The optimal operating condition was obtained applying all alcohol/oil molar ratio of 9.0 and a catalyst concentration of 3.5% (w/w) H2SO4.