It was recently reported that poly(ethylene glycol) (PEG) additives (PEG and its hybrids with inorganic fillers) could significantly improve the processability of ultrahigh-molecular-weight polyethylene (UPE)/polypropylene (PP) blends. The influence of PEG additives on the mechanical properties of UPE/PP blends was investigated in this study. With the addition of small amounts of PEG additives, the tensile properties of UPE/PP blends were little affected, whereas the loss in impact strength was relatively noticeable; however, the toughness was maintained at a high level. Stress-relaxation and stress-strain tests showed that the PEG additives acted mainly in the amorphous phase, reducing the entanglement density of the blends. (C) 2008 Wiley Periodicals, Inc.