Abstract: Low-power-density ultrasonic transesterification of soybean oil with methanol was studied. It was thought that biodiesel with good quality could be obtained under the condition of lower power density of ultrasound. Molar ratio of soybean oil to methanol was 1:6. Mass ratio of potassium hydroxide to soybean oil was 0.01:1. Reaction temperature was about 30℃. Reaction time under ultrasound was 20-60 min according to the reaction scale. Conversion of transesterification calculated from refractive index of biodiesel was 84.81%-85.85%. The lowest power density used in this study was about 0.032 W/cm3 which is far lower than that of the traditional 0.5 W/cm3.