In this study, graphene-nanoplatelet (GNP) and carbon-nanotube (CNT) reinforced nanocomposites were produced with pressure molding. 1 w/% of GNPs and 1 w/% of CNTs were separately added to a polyethylene (PE) matrix and 0.5 w/% of both reinforcements was also jointly added to the PE matrix. In this manner, the effects of GNPs and CNTs on the mechanical properties of PE were compared and the synergistic effect was investigated. In order to examine the mechanical properties, a tensile test, a hardness test and a wear test were applied to the produced samples. Also, fracture surfaces and wear surfaces were investigated with a scanning electron microscope (SEM). It was observed that the graphene was distributed homogeneously in the polyethylene matrix. Thus, the GNP-containing samples showed better mechanical properties than the CNT-containing samples. Keywords: polymer-matrix composites, compression molding, high-density polyethylene, graphene, carbon nanotube V {tudiji avtorji opisujejo izdelavo polimernih kompozitov, oja~anih z grafenskimi nanoplo{~icami (GNPs) in ogljikovimi nanocevkami (CNTs). Postopek izdelave je potekal z oblikovanjem v kovinskem modelu pod tlakom. V prvo polietilensko (PE) matrico so dodali 1 mas. % GNPs, v drugo 1 mas. % CNTs in v tretjo po 0,5 mas. % obeh oja~itvenih faz. Nato so dolo~ili mehanske lastnosti izdelanih kompozitov in ugotavljali vpliv dodatka GNPs in CNTs ter raziskovali sinergijski u~inek obeh. V ta namen so na izdelanih preizku{ancih izvedli natezni preizkus, meritve trdote in odpornosti proti obrabi. Prelomne in obrabne povr{ine so prav tako pregledali pod vrsti~nim elektronskim mikroskopom (SEM). Rezultati raziskave so pokazali, da je grafen enakomerno porazdeljen po PE matrici. Zato so imeli vzorci, ki so vsebovali GNPs bolj{e mehanske lastnosti kot vzorci, ki so vsebovali CNTs. Klju~ne besede: kompoziti s polimerno matrico, oblikovanje pod tlakom, polietilen z visoko gostoto, grafen, ogljikove nanocevke