The formation of core-shell morphology within the dispersed phase was studied for composite droplet polymer-blend systems comprising a polyamide-6 matrix, ethylene-propylene-diene terpolymer (EPDM) shell and high density polyethylene (HDPE) core. In this article, the effect of EPDM with different molecular weights on the morphology and properties of the blends were studied. To improve the compatibility of the ternary blends, EPDM was modified by grafting with maleic anhydride (EPDM-g-MAH). It was found that core-shell morphology with EPDM-g-MAH as shell and HDPE as core and separated dispersion morphology of EPDM-g-MAH and HDPE phase were obtained separately in PA6 matrix with different molecular weights of EPDM-g-MAH in the blends. DSC measurement indicated that there may be some co-crystals in the blends due to the formation of core-shell structure. Mechanical tests showed that PA6/EPDM-g-MAH/HDPE ternary blends with the core-shell morphology exhibited a remarkable rise in the elongation at break. With more perfect core-shell composite droplets and co-crystals, the impact strength of the ternary blends could be greatly increased to 51.38 kJ m−2, almost 10 times higher than that of pure PA6 (5.50 kJ m−2). POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers