Reactive extrusion of polypropylene (PP)/ethylene-propylene-diene terpolymer (EPDM)/high-density polyethylene (PE) (80/10/10 by weight) blends were carried out using a corotating twin-screw extruder. The effects of peroxide and coagent concentrations and extruder rpm were studied in terms of rheological, morphological, thermal, and mechanical properties of the blends. Melt viscosity of the peroxide-treated blend increased and decreased over the untreated one depending on the amount of a coagent. Morphologically, interfaces blur with only a peroxide treatment, and significant domain reduction was obtained when peroxide and a coagent were used together. Both T-m (crystalline melting temperature) and T-g (glass transition temperature) of PP increased in the blend, whereas those of PE slightly decreased.