In preparing a non-polar polymer composite employing functional fillers, the chemical inertness of non-polar polymer does not easily allow efficient interfacial affinity change. Here, a wet-free interfacial affinity modulation to incompatible polypropylene (PP)/graphite nanoplatelet (GNP) composite was performed with a nitrogen plasma-assisted mechanochemical (PM) treatment. It was surprising that the PP/GNP composite subjected to the PM treatment demonstrated the formation of chemical bonds between PP and GNP and the dramatic improvement of GNP dispersion in PP, resulting in a novel PP composite deploying efficient heat transfer along with mechanical performance comparable to that of neat PP. Due to the interfacial affinity control, the elongation at break of the PM-treated PP/GNP composite with 5 wt% of GNP was improved ten-folds compared to the untreated PP/GNP composite. The thermal conductivity of the PM-treated PP/GNP composite employing 20 wt% of GNP reached ca. 21 W/mK, whereas the untreated PP/GNP composite containing comparable amount of GNP exhibited 7 W/mK. Based on the findings, it can be noted that a solvent-free plasma-assisted mechanochemical treatment could be meaningful in developing a novel composite capable of efficient mechanical load transfer and heat transfer with non-polar polymers known to be poor at chemical reaction.