Abstract:Soybean oil was epoxided by the assist of ultrasonic in the presence of formic acid a the carrier of active oxygen and stearic acid  as the catalyst.The parameters affecting the epoxy value of soybean oil such as the amount of formic acid,stearic acid and hydrogen perocide,as well as the ultrasonic frequency,power,temperature and time were discussed.It found that the epoxy value of epoxidized soybean oil(ESBD) reached 5.93% under the optimum conditions of amount of formic acid 20%,stearic acid 2.0%,molar ratio of hydrogen peroxide to formic acid 7.76:1, ultrasonic  frequency 28kHz, power 120W, temperature 65¡æ, ultrasonic time 30 min.The epoxy value of epoxidized soybean oil treated by ultrasound increased  by 17.19% compared with that treated by non-ultrasound in the same reaction time, and the epoxidation time reduced nearly half compared with that treated by non-ultrasound in the same epoxy value.