In this paper, high power ultrasound was introduced into polystyrene (PS)/ethylene-propylene diene monomer (EPDM)(80/20) blend melts during extrusion. The structure and properties of PS/EPDM blends treated ultrasonically, such as their mechanical properties, phasemorphology, dynamic rheological behavior and the size distribution fractal dimension of dispersed particles, as well as the interfacial tension between the PS and EPDMphases, were studied. The experimental results indicated that ultrasonic treatment could improve the compatibility of PS/EPDM blends. This is attributed to the formation of a copolymer of PS and EPDMby the combination of different macroradicals, resulting from the homolytic cleavage of polymer chains induced by ultrasonic irradiation. The disproportional termination of the macroradicals leads to the degradation of PS and EPDM. Some properties, like mechanical and linear viscoelastic characteristics, were affected by both ultrasonic compatibilization and degradation. These properties depended strongly on the ultrasonic intensity, ultrasonic treatment time and number of extrusions (repeated extruded times). Differential particle size distribution of the dispersed phase in PS/EPDM blends.