To better understand the Izod impact strength of modified poly(methyl methacrylate-co-styrene), the fracture mode and whitened state of molded specimens were studied. The Izod impact strength was closely related to the amount of whitened area, and the degree of whiteness improved with increasing whitened area and degree. The whitened area was strongly related to the transition rate for shear yielding in tensile testing. The degree of whitening was strongly related to the elongation at break in low-speed tensile testing. Transmission and scanning electron microscopy were also used to develop an understanding of the mechanisms of energy absorption and the fracture process.