The effect of orientation hardening of matrix polymers on the toughness of polymer blends was examined. Both poly(methyl methacrylate) (PMMA) and poly(vinyl chloride) (PVC), with different characteristics of orientation hardening were used as matrix polymers and a silicone/acrylic composite rubber graft copolymer was used as the modifier. The efficiency of the improvement in toughness for the blend with PMMA as the matrix polymer, as a result of the addition of the modifier, was lower than that of the blend with PVC. It was suggested that the strain for the initiation of orientation hardening of PMMA. was lower than that of PVC. It was concluded that the lower efficiency of toughening for PMMA blended with modifier when compared to that of the PVC blend is caused by the development of orientation hardening at a lower plastic strain.