The present study deals with the development of a combined reactor involving an ultrasonic reactor (UR) and a mechanical stirrer (MS) for the transesterification of vegetable oil, extracted from semal (Bombax Ceiba). Reaction variables such as reaction time, methanol to oil molar ratio, catalyst concentration, and ultrasonic irradiation power were investigated to find the optimal parameters for maximizing biodiesel yield. The optimum conditions with the combined process reactor (CPR) are: 30 min reaction time, 4.5/1 molar ratio, 0.5% catalyst, and 40% of the maximum ultrasonic power with a maximum yield of 96.4% as compared to 110 min, 6/1 molar ratio, and 1% catalyst with a maximum yield of 90.7% for the MS, and 40 min, 4.5/1 molar ratio, 0.75% catalyst, and 50% of the maximum ultrasonic power with a maximum yield of 92.1% in a UR.