The effects of ultrasonic irradiation on the mechanical properties, morphology, and crystal structure of polypropylene (PP)/ethylene-propylene-diene terpolymer (EPDM) blends were examined. Results show that appropriate irradiation intensity can noticeably increase the toughness of the PP/EPDM blends without reducing rigidity. Scanning electron microscopic (SEM) observation shows that with ultrasonic irradiation, the morphology of a well-dispersed EPDM phase is formed in the PP/EPDM blend. The glass transition temperatures of PP and EPDM phase approach each other as a result of ultrasonic irradiation. Differential Scanning Calorimetric (DSC) analysis indicates that the crystallinities of the PP and EPDM phases increase with ultrasonic irradiation, and beta crystals of PP form in the PP/EPDM blend with ultrasonic irradiation, which is proven by wide angle X-ray diffraction (WAXD) analysis. (C) 2004 Society of Plastics Engineers.