In order to improve the physical and mechanical properties of specimens of high-impact polystyrene, a graft copolymer of atactic polypropylene with styrene was introduced into its composition. The graft copolymer is distinguished by the fact that there is no homopolymer of the monomer being grafted, i.e. polystyrene, in its composition. It was found that, depending on the length and distribution frequency of the grafted chains, the introduction of the graft copolymer helps to improve the breaking stress, bending strength and impact strength of the composite material.