Our objective was to determine the effect of ultrasonics on biodiesel production from soybean oil. In this study, ultrasonic energy was applied in two different modes: pulse and continuous sonication. Soybean oil was mixed with methanol and a catalytic amount of sodium hydroxide, and the mixture was sonicated at three levels of amplitude (60, 120, and 180 μm_pp) in pulse mode (5 s on/25 s off). In the continuous mode, the same reaction mixture was sonicated at 120 μm_pp for 15 s. The reaction was monitored for biodiesel yield by stopping the reaction at selected time intervals and analyzing the biodiesel content by thermogravimetric analysis (TGA). The results were compared to a control group, in which the same reactant composition was allowed to react at 60 °C for intervals ranging from 5 min to 1 h without ultrasonic treatment. It was observed that ultrasonic treatment resulted in a 96% by weight isolated yield of biodiesel in less than 90 s using the pulse mode, compared to 30−45 min for the unsonicated control sample with comparable yields (83−86%). In the pulse mode, the highest yield (96%) was obtained by sonicating the mixture at 120 μm_pp amplitude. In the continuous sonication mode, the highest biodiesel yield was 86% by weight, which was obtained in 15 s.