The additive effects of a diblock copolymer of polypropylene and ethylene-propylene rubber PP-b-EPR(50-50) on the morphology and mechanical properties of PP/EPR(50/50) blends were investigated. It was found from an electron microscopy study that the block copolymer is compatible with the PP phase but incompatible with the EPR phase. Dynamic mechanical spectra showed that the PP-b-EPR block copolymers are incorporated into the PP phase, and consequently, the EPR portions of the block are trapped into the interlamellae of PP.