In this study, process intensification was carried out using lipase catalyzed transesterification of waste cottonseed oil (WCSO) with ethanol in the presence of ultrasonication. A maximum conversion of 98.7 was observed for ultrasound assisted transesterification. The optimum process conditions were ethanol to oil molar ratio of 4.5:1, a reaction temperature of 45°C, enzyme loading of 5 wt, a reaction time of 6 h, ultrasonic amplitude of 40 and the duty cycle of 15 s ON and 15 s OFF. The influence of ultrasound makes the process efficient because of an effective reduction in reaction time to 6 h with the least catalyst dosage and power consumption. Reusability tests of the catalyst were conducted after separating the immobilized Rhizopus oryzae lipase. The regenerated lipase catalyst was found to have better reusability up to the fourth cycle and the produced biodiesel was within the ASTM D6751 standard. Furthermore, the ultrasound assisted process under mild operating conditions did not affect the immobilized enzyme. It was clearly observed that ultrasonication is faster and effective for biodiesel conversion using the immobilized lipase catalyzed transesterification reactions. © 2020 Elsevier B.V.