ABSTRACTThis study deals with evaluation of mechanical properties (tensile, flexural and impact strength), that verified reinforcing potential of nanofibres in polymeric composites and their applicability in dependence on the filler content of nanofibres of natural and synthetic origin. Considering the hydrophilic nature of natural nanofibres and hydrophobic polypropylene matrix was applied chemical modification that ensures effective interlock of nanofibres with the matrix, namely maleic anhydride grafted polypropylene and ionic modifier. Polymeric nanocomposites were prepared by twin screw extrusion that made possible mixing of all three components together. After cooling in water bath the composites were cut on pellets and dried for further injection moulding. The specimens were made of two- or three-phase component systems that included PP matrix, coupling agent in the volume of 4 wt.% and reinforcing nanofillers in the volume of 2, 4 and 6 wt.%. The presence of nanoparticles and coupling agents had not unambiguous impact on analysed mechanical properties. Coupling agent presence was dominant for impact strength, however, flexural and tensile properties were influenced mainly by the nanofiller type and origin. The dispersed phase and compatibilizer improved flexural modulus but tensile modulus of pure PP were not achieved by three-phase composite, not even with the highest analysed nanoparticle load. Cellulose fibres proved as more proper than the synthetic ones not only for their biodegradability and renewability but for their better toughness as well.