In this work, phase morphology and mechanical properties of ternary blends of PA6 (Polyamide 6), EPDM-g-MA (Ethylene-propylene-diene rubber grafted with maleic anhydride) and different HDPEs (high-density polyethylene) have been studied. Although HDPEs of various viscosity and contents are used, the binary (EPDM-g-MA/HDPE) dispersed phase is of a core-shell structure, in which HOPE forms the core. When low viscosity HOPE is used, the blends consist of single-core core-shell structure. While, the application of high viscosity HDPE plays an important role in forming a multinuclear core-shell structure. Furthermore, the notched Izod impact strength of PA6/EPDM-g-MA/H-HDPE blends 70/15/15 reaches the highest, which is 2 times higher than PA6/EPDM-g-MA 70/30 binary blends. The study of the toughening mechanism of PA6 ternary blends with core-shell structure suggest that the fibrillation of core-shell particles in PA6/EPDM-g-MA/HDPE5 ternary blends can absorb fracture impact energy and prevent micro-crack further propagating, and thus obtain higher notched Izod impact strength. (C) 2014 Elsevier Ltd. All rights reserved.