Supramolecular block copolymers comprising isotactic polypropylene (iPP) and ethylene-propylene random copolymers (EP) with complementary quadruple hydrogen bonding junctions have been prepared by melt-mixing of iPP having a 2-ureido-4[1H]-pyrimidinone (UPy) group (iPP-UPy) and elastic EP bearing a 2,7-diamido-1,8-naphthyridine (Napy) group (EP-Napy). Transmission electron microscope (TEM) analysis of the iPP-UPy/EP-Napy composite showed that the elastic EP domains were well dispersed in the iPP matrix compared with the traditional iPP/EP impact polypropylene copolymer (IPC). The iPP-UPy/EP-Napy hydrogen-bonded pseudo block copolymer effectively acts as a compatibilizer in the IPC and contributes to improved mechanical properties of the resulting iPP/EP composite. There is good correlation between impact strength of the IPC and EP domain size observed by the TEM analysis. The use of the complementary quadruple hydrogen bonding system for blending two immiscible polymers has been shown to result in smaller domain sizes of the EP-phase in the iPP and consequently improved mechanical properties of the supramolecular iPP/EP blends.