The high density polyethylene-polyhedral oligomeric silsesquioxane (HDPE-POSS) organic-inorganic hybrids were obtained and their properties studied. The hybrids were prepared by grafting of POSS on polyethylene chains during reactive melt-blending of HDPE, maleic anhydride functionalized HDPE and amine-functionalized POSS, taking advantage of the high efficiency of amine-anhydride reaction in the molten state. The structure, morphology, and physical properties of the obtained hybrids and blends were studied to find the influence of POSS chemical structure and grafting degree on the morphological characteristics and mechanical properties of hybrid nanocomposites. It was found that grafting of POSS cages on HDPE chains leads to the POSS dispersion at the molecular level. On the contrary, when POSS was mixed with plain HDPE any grafting of POSS on polyethylene chains was not possible, which resulted in phase-separated blend. The mechanical tests revealed that modification of polyethylene by grafting with POSS molecules does not affect significantly its mechanical properties, both static and dynamic, except ultimate strain, which is lower in hybrids and their blends than in plain HDPE. The impact properties (Izod impact strength) were slightly improved by grafting of HDPE with POSS. HDPE-g-POSS hybrids demonstrated also much enhanced thermo-oxidative stability comparing to plain polyethylene. POLYM. ENG. SCI., 55:2058-2072, 2015. (c) 2014 Society of Plastics Engineers