The toughness of a plastic can be enhanced substantially by blending with rubber particles [1]. Both morphological parameters and interfacial adhesion between the matrix and the dispersed particles play important roles in controlling the toughness of the blends. It is commonly believed that the effects of morphological parameters and interfacial adhesion are interrelated. For instance, strengthening the interfacial adhesion leads to a better dispersion of particles and smaller particle size and hence a change of toughness. This effect of interfacial adhesion on toughening is termed indirect [2]. On the other hand, interfacial adhesion may have a direct effect on toughening when the morphological parameters are identical