Polystyrene (PS) was blended with linear low density polyethylene (LLDPE) and with elastomer styrene-ethylene-butylene-styrene (SEBS) terpolymer. The effects of the difference of dispersed phase on mechanical properties of the blends were investigated. The breaking elongation of the PS/SEBS blends was increased with increasing SEBS content. However, the breaking elongation of PS/LLDPE blends was not increased with increasing LLDPE content. This reason was studied by using the stress analysis of Finite Element Method. Little difference was found between the dilatational stress distribution in PS around SEBS particles and that around viods. This means that the craze initiation is not an important factor to control the breaking elongation of the blends. Therefore, the propagation mechanism of the craze in PS is considered to be influenced by the kind of dispersed phase. This is responsible for an increase in breaking elongation of PS/SEBS blends.