The effect of the volume fraction of a dispersed phase on improvement in the toughness of injection-molded isotactic polypropylene (i-PP), blended with a styrenic thermoplastic elastomer, has been studied. The volume fraction of the dispersed phase, which indicates compatibility between elastomer and i-IPP, depends on the molecular structure of a soft segment and the copolymerization ratio of styrene. The volume fraction of the dispersed phase of the i-PP blended with styrene-ethylene-propylene-styrene tri-block copolymer (SEPS) was higher than that of the i-PP blended with styrene-ethylene-butylene-styrene tri-block copolymer (SEBS). The volume fraction of the dispersed phase increases as the copolymerization ratio of styrene increases. Improvement in the toughness of the blended i-PP clearly depends on relaxation of strain constraint by void formation from the dispersed phase. Its efficiency proved to be dependent on the strength and the volume fraction of the dispersed phase depending on the compatibility between elastomer and i-PP.