[en] Polypropylene (PP)/high-density polyethylene (HDPE)/ethylene propylene diene terpolymer (EPDM) based ternary blends and their composites with graphene nanoplatelets (xGnP) were prepared. The effect of EPDM and both EPDM and xGnP on the thermo-mechanical properties of the PP/HDPE blends were systematically studied. Scanning electron microscopy (SEM), mechanical measurements, dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) of the PP/HDPE/EPDM ternary blends and PP/HDPE/EPDM/xGnP ternary composites were conducted. SEM micrographs reveal an excellent compatibility of EPDM with PP/HDPE blends. The presence of EPDM in the PP/HDPE (80/20) blends enhanced the impact strength and elongation at break at the expense of tensile strength and modulus. The morphology of the ternary composites reveals an interpenetrated complex morphology. The presence of xGnP in the ternary blends had greatly improved the tensile toughness of the composites. The storage modulus and loss modulus of the PP/HDPE blends was reduced by the addition of EPDM elastomer. On the other hand, the presence of xGnP improved the storage modulus and loss modulus of the ternary composites. The melting temperature and crystalline temperature of both the PP phase and HDPE phase remains stable with the addition of EPDM and xGnP, while the percentage crystallinity was increased. (paper)