To compare oil-in-water emulsification behaviors between ultrasonic irradiation and impeller stirring, experiments and calculation by mathematical models were performed. For ultrasonic irradiation, the peak diameter of dispersed oil droplets decreased with an increase in input power and reductions in ultrasonic frequency, ratio of dispersed volume, and interfacial tension. For mechanical stirring, it declined with higher mechanical rotation speed. The results calculated by the mathematical models approximately agreed with the experiments. Ultrasonic irradiation was better than mechanical stirring by an impeller for producing a fine emulsion at higher supplied energy rate into liquid, whereas the tendency was inverse for a rough emulsion at lower input energy rate.