The objective of this study is to examine the influence of poly [styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) compatibilizer on the morphology and properties of atactic polystyrene/high density polyethylene (aPS/HDPE) blends. The rheological behavior of the blends melt during processing is followed. The concentration of SEBS does not significantly influence the processing parameters. The morphology is determined by SEM and TEM. aPS/HDPE/SEBS blends exhibit fine dispersion of HDPE or aPS particles in the matrix and better adhesion at the interface. Higher concentration of SEBS results in better compatibility. Transmission electron micrographs confirm existence of the SEBS interfacial layer between the aPS and the HDPE phase. This effect is stronger in the blends with higher SEBS concentration and HDPE as matrix. The mechanical properties are determined. The tensile strength decreases while the elongation at break and the impact strength increase with the SEBS content. The results prove that SEBS acts as a compatibilizer in aPS/HDPE blends and its effect is more expressed in blends with a higher SEBS content and HDPE as matrix.