The toughening mechanisms of rubber-dispersed polymer alloys were investigated with respect to two factors : (i) characteristic ratio, C(infinity), as a measure of chain flexibility of the polymer matrix and (ii) the rubber particle size in high impact polystyrene/poly(2,6-dimethyl-1,4-phenylene ether) blend systems. Measurements of the specific gravity, synchrotron radiation small angle X-ray scattering (SR-SAXS) tests, and finite element analysis were carried out to gain understanding of plastic deformation (crazing and shear yielding) of these materials. Shear yielding was found to be enhanced when the C(infinity) value of the matrix polymer was relatively low and the rubber particles were small (submicron). From these results, we presumed the impact energy absorption mechanisms to be governed by these two factors.