The structure and properties of the dynamically cured blends of the high density polyethylene(HDPE) and ethylene-propylene-diene terpolymer(EPDM) were studied. The dynamically cured blends were prepared using an annular curing die with shear generated by the rotation of the inner cylinder. EPDM was cured with dicumyl peroxide(DCP) in the presence of HDPE under different shear rates. The effects of shear rate, DCP concentration, and HDPE/EPDM composition on the morphology and the rheological, and mechanical properties of the dynamically cured HDPE/EPDM blends were studied. The rheological and mechanical properties of the blends were investigated by capillary rheometer, Rheometrics dynamic spectrometer, tensile tester, and durometer. The morphology of the blends was observed by the optical microscope, scanning electron microscope (SEM), and transmission electron microscope(TEMO. The processing characteristics during the continuous curing reaction was also investigated. All of the dynamically cured blends prepared in this study could be fabricated by injection molding due to the characteristic morphological feature even though there existed crosslinked rubber as one component. The blends showed greatly improved properties in comparison with the uncured blends.