Abstract: The application of ultrasound in the viscosity reduction of vacuum residuum was investigated. Effects of ultrasonic treatment time，output voltage and temperature on the viscosity reduction ratio etc. were studied. Results showed that the higher of output voltage and the longer of ultrasonic treatment time，the higher the viscosity reduction ratio. When the ultrasonic treatment temperature is lower than 280℃，the viscosity reduction ratioes at 40℃ are less than 30% and 10％ respectively with and without ultrasonic treatment，and the viscosity reduction ratio at 80℃ are both less than 5% with and without ultrasonic treatment. The main effects of ultrasonic treatment below 280℃ for viscosity reduction might be mechanical agitation and physical changes in vacuum residuum. When the ultrasonic treatment temperature is from 300℃ to 400℃，the viscosity reduction ratio at 40℃ is from 34% to 63％ and from 13％ to 34％ with and without ultrasonic treatment，and the viscosity reduction ratio at 80℃ is from 7% to 20％ and from 3％ to 10％ with and without ultrasonic treatment，and the viscosity of vacuum residuum is stable within 9 days. The main effects of ultrasonic treatment between 300 and 400℃ might be the cavitation visbreaking，which leads to the viscosity reduction of the cracked residuum.