Abstract: Transesterification of rice bran oil with alcohol, in presence of base catalyst, was studied with low frequency ultrasound, as well as the discussion of the feasibility of biodiesel preparation with ultrasonic assistant. By using ultrasounds, the reaction time was reduced 15～20 min compared to mechanical stirring. The reaction rate of the 40 kHz ultrasound was faster than that of 28 kHz but lower yield than 28 kHz ultrasound. The quantity of required catalyst can be reduced with ultrasonic assistant. The best yields were obtained when the catalyst was used in 0.5％ wt/wt of oil. The differences in the product yield were mainly due to difficulties through washing. Emission tests have shown a marked decreasing in CO, HC, and PM with a slight increasing in NOx, however.