The facile approach of non-covalent surface treatment utilizing poly(ethyleneimine) (PEI) polymer was applied to modify graphene nanoplatelets (GNPs). The effects of surface modification and various GNPs-PEI loadings to cure characteristic, mechanical, physical, and morphological properties of natural rubber (NR)/ethylene-propylene-diene-monomer (EPDM) blend nanocomposites were studied and compared to the unfilled NR/EPDM blend and blends filled with unmodified GNPs at similar loadings. We found that the modification of GNPs surface significantly influences the properties of NR/EPDM blends. The addition of GNPs significantly improved the blend's processability, offering approximately a 104.30 % increase in tensile strength obtained with the addition of 5.00 wt% GNPs-PEI. A reduced swelling index of Q(f)/Q(g) in parallel with an increase in modified GNPs-PEI content revealed enhancements in terms of rubber-filler interactions between the NR/EPDM matrix and GNPs. These findings were further supported by X-ray diffraction, differential scanning calorimetry, thermogravimetry analysis, and fracture morphology by scanning electron microscope.