The effect of molecular structure of six model styrene-butadiene (SB) block copolymers with various number of blocks and two lengths of styrene blocks on morphology, rheological properties, and impact strength of polystyrene (PS)/high-density polyethylene (PE) blends was studied. It was found that location of SB copolymers in the blends is determined by the length of styrene blocks. The length of styrene blocks has similar effects on impact strength and linear viscoelastic properties of the blends. On the other hand, the correlation was not found between the effects of a number of blocks on impact strength and linear viscoelastic properties of the blends. (C) 2003 Wiley Periodicals, Inc.