The effects of the addition of diblock copolymer poly( styrene-b-ethylene-copropylene) (SEP) to isotactic polypropylene (iPP) on the morphology and mechanical properties were investigated. Phase morphologies of iPP/SEP blends up to a 70/30 weight ratio, prepared in Brabender Plasticoder, were studied with optical microscopy, scanning electron microscopy, transmission electron microscopy, and wide-angle X-ray diffraction. The addition of 2.5 wt % SEP caused a nucleation effect (by decreasing the crystallite and spherulite size) and randomization of the crystallites. With further SEP addition, the crystallite and spherulite size increased because of prolonged solidification and crystallization and achieved the maximum in the 80/20 iPP/SEP blend. This maximum was a result of the appearance of beta spherulites and the presence of mixed alpha spherulites in the 80/20 iPP/SEP blend. Dispersed SEP particles were irregular and elongated clusters consisting of oval and spherical core-shell microdomains or SEP micelles. SEP clusters accommodated their shapes to interlamellar and interspherulitic regions, which enabled a well-developed spherulitization even in the 70/30 iPP/SEP blend. The addition of SEP decreased the yield stress, elongation at yield, and Young's modulus but significantly improved the notched impact strength with respect to the strength of pure iPP at room temperature. Some theoretical models for the determination of Young's modulus of iPP/SEP blends were applied for a comparison with the experimental results. The experimental line was closest to the Takayanagi series model. (C) 2001 John Wiley & Sons, Inc.