The immiscible polypropylene (PP)/natural rubber (NR) blends of various concentrations were prepared by using a twin-screw extruder. The prepared blends were passed through the reactor where they were ultrasonically treated by an extrusion process. Mechanical properties and theology of the obtained blends were studied, along with morphology by using the scanning electron microscopy and the atomic force microscopy (AFM). Mechanical properties of the treated blends were found to improve significantly in comparison with those of untreated blends. Under most treatment conditions, no significant differences in the viscosity of the treated and untreated blends were observed. The AFM studies revealed the development of interfacial layers, interfacial roughening and improved interfacial adhesion between PP and NR phases in the blends subjected to ultrasonic treatment. At the same time weak adhesion and delamination at the interface were found in the untreated blends. The improved interfacial adhesion, morphology and mechanical properties are believed to be due to the formation of in situ copolymer at the interface of two immiscible polymers caused by an ultrasonic treatment without the use of any chemicals. (C) 2003 Elsevier Science Ltd. All rights reserved.