Toughening mechanisms of three types of rubber toughened poly(methyl methacrylate)(RT-PMMA) were investigated under mode I loading condition by optical and electron microscopies in conjunction with the quantitative evaluation of mode I fracture toughness. Polarizing optical microscopy clearly exhibited damage zone development ahead of a crack-tip in the RT-PMMA's. The three RT-PMMA's revealed different shapes of the damage zone. Transmission electron microscopy exhibited microcrazes generated in the equator of rubber particles within the damage zones. Extensive deformation of rubber particles corresponding to localized shear yielding of the PMMA matrix was also found in a region close to a propagating crack-tip. In addition, cavitation of rubber particles was observed in the vicinity of the crack. It is therefore understood that the toughening of the RT-PMMA's is due to energy dissipation caused by the microdamage formations such as microcrazing, matrix shear deformation and rubber particle cavitation ahead of the crack-tip.