In this study, ethylene/ styrene interpolymer was used as a compatibilizer for the blends of polystyrene (PS) and high-density polyethylene (HDPE). The mechanical properties including tensile and impact properties and morphology of the blends were investigated by means of uniaxial tension, instrumented falling-weight impact measurements, and scanning electron microscopy. Tensile tests showed that the yield strength of the PS/HDPE/ESI blends decreases considerably with increasing HDPE content. However, the elongation at break of the blends tended to increase significantly with increasing HDPE content. The excellent tensile ductility of the HDPE-rich blends resulted from shield yielding of the matrix. Izod and Charpy impact measurements indicated that the impact strength of the blends increases slowly with HDPE content up to 40 wt %; thereafter, it increases sharply with increasing HDPE content. The impact energy of the HDPE-rich blends exceeded that of pure HDPE, implying that the HDPE polymer can be further toughened by the incorporation of brittle PS minor phase in the presence of ESI compatibilizer. The correlation between the impact property and morphology of the blends is discussed. (C) 2007 Wiley Periodicals, Inc.