The Izod impact strength of poly(vinyl chloride)/methylmethacrylate/butadiene/ styrene(PVC/MBS)polymer blends can be changed significantly with different levels of MBS and/or MBS particle size. The following results were obtained by investigating the fracture of PVC/MBS test specimens : 1) The dependence of the Izod impact strength of PVC/MBS blend on MBS particle size confirms a maximum around a MBS particle size of 2000 Angstrom. When MBS particle size is smaller than 2000 degrees, the Izod impact strength increases with MBS particle size, and crazing occurs mainly in this region. When MBS particle size is larger than 2000 Angstrom, then the Izod impact strength, in contrast, decreases with increasing MBS particle size, and both crazing and shear yielding occur, mainly in this region. 2) Tensile experiments of PVC/MBS blends carried out under various conditions showed that the amount of energy absorption increases with decreasing MBS inter-particle distance and with increasing MBS particle size when crazing is the main energy absorbing mode. The MBS inter-particle distance dominates the energy absorption when shear yielding is the main energy absorbing mode. 3) Therefore, the Izod impact strength of PVC/MBS blends and the maximum around a MBS particle size of 2000 Angstrom can be explained as follows : Below 2000 Angstrom, the energy absorption by crazing dominates the total energy absorption, and the energy absorption by crazing increases with MBS particle size. Above 2000 Angstrom, the energy absorption by shear yielding is dominant, and the energy absorption by shear yielding increases with decreasing inter-particle distance, that is to say, decreasing MBS particle size.