In order to study the effect of particle size on the toughening behaviour of rubber-toughened poly(methyl methacrylate) (PMMA), a systematic model study has been carried out using core-shell type particles, made up of a poly(n-butyl acrylate) (PBA) core and a PMMA outer shell. Two different fracture tests, i.e. a three-point bending test for the evaluation of the fracture toughness (K-1C) and an impact test were employed to study the toughening behaviour. In the case of the impact test, maximum impact strength was obtained for rubber particles with a 0.25 mu m diameter regardless of the rubber phase contents, and only a modest improvement of the impact strength was obtained for blends containing 0.15 or 2 mu m particles. For the three-point bending test, in contrast, even large particles such as the 2 mu m particles provided a significant enhancement in the fracture toughness. The difference in the toughening behaviour due to particle size may be attributed to the test method dependence associated with a change in the deformation mechanism. In the three-point bending test, the deformation mechanism was found to be multiple crazing, whereas in the impact test, the shear yielding induced by cavitation of the rubber particles is predominant.