A significant improvement of the impact strength for poly(methylmethacrylate) (PMMA) alloy was obtained by using an acrylic rubber modifier containing a small amount of silicone or a butadiene/butylacrylate (Bd/BA) aggregation type of modifier. For the modifier containing silicone, a remarkable small angle X-ray scattering (SAXS) could be observed, and a small sized submicrocracks (SMCs) appeared inside the rubber particle under the deformation. It has been suggested that the small amount of silicone contained in the rubber might play a role in the production of nuclei of SMC origin. Therefore, even if the amount of silicone content was quite small, it was effective enough to generate the SMCs. Thus, the function of the silicons is to improve toughness in view of inducing void formation. The Bd/BA-aggregation type of modifier reaches fracture even under small stress, and followed by jumping suddenly to a cavity far larger than the detection limit of our SAXS apparatus at an early stage of strain. This fracture mechanism is completely different from that of the silicone type.