The influence of blend composition and ethylene–propylene–diene terpolymer (EPDM) content on the processing parameters, phase structure and thermal behaviour of polypropylene (PP) and low-density polyethylene (LDPE) blends was studied. Processing parameters of blends in twin-screw extruder were followed by output (Q), torque (TQ) and back pressure (p), and apparent viscosity as given by TQ/Q and p/Q ratios, while the thermal behaviour was measured using differential scanning calorimetry. Phase structure was investigated using scanning transmission electron microscopy (STEM). The degree of crystallinity ( _c) of PP phase decreased and _c of LDPE phase increased with higher LDPE content in blends with and without EPDM. STEM micrographs of EPDM compatibilized PP/LDPE blends of compositions 80/20 and 20/80 showed particulate morphology, whereas in the blends of compositions 60/40 and 40/60 the morphology was co-continuous. The EPDM compatibilizer was localized on the interface.