Biodiesel is an alternative renewable fuel in the world energy matrix, originating from animal fats and vegetable oils from grains, such as the castor bean cultivated by family farmers in the Brazilian semiarid region. However, the production of biodiesel has some disadvantages. For example, it is difficult to use the same types of oil/fat all year round. In addition, the use of methanol in the biodiesel synthesis and agitation process results in a high consumption of energy. Given this background, this article proposes an alternative biodiesel synthesis using castor oil and peanut oil mixtures, ultrasound-assisted in an ethylic route. The experimental conditions were optimized utilizing a Box-Behnken design. The response variable was a biodiesel yield. In this work, it was possible to reduce the ratio of alcohol to oil and thus reduce the energy consumption and increase the mass yield. This permitted the generation of biofuel with lower production costs. Finally, the response surface methodology leads to operational conditions more suitable for the production of biodiesel. In this work, the best conditions were obtained using 0.50% catalyst concentration, 50/50 castor:peanut oil rate, and 5:1 alcohol:oil ratio. These were moderate and suitable conditions for the synthesis of biodiesel in comparison with the works found in the literature, reaching a high yield of 97.93%.