The effects of ultrasonic irradiation on extrusion processing and crystallization behavior of polypropylene (PP) are examined. The results show that appropriate irradiation intensity can prominently decrease die pressure and apparent viscosity, increase output, reduce extrudate swell, and improve apparent quality of PP. DSC and WAXD analysis demonstrate that the crystalline structure of PP is changed and the content of the beta crystalline increases through ultrasonic vibration. Special effects can be induced by ultrasound, such as strong stress, shatter and vibration, which alter the growth of PP crystals and may favor a special crystal transformation (alpha --> beta).