The effect of matrix compositions on the toughening of bismaleimide resin by modification with N-phenylmaleimide-styrene copolymers (PMS) were examined. The bismaleimide resin was composed of 4,4’-bismaleimidediphenyl methane (BMI), o,o’-diallyl bisphenol A (DBA), and triallyl isocyanurate (TAIC). The matrix structure was controlled by changing the equivalent ratio of the two allyl components (DBA and TAIC). Morphologies of the modified resins changed from particulate to cocontinuous and to inverted phase structures, depending on the modifier content. The most effective mod ification for the cured resins could be attained because of the cocontinuous structure of the modified resins. Inclusion of TAIC led to a decrease in the extent of dispersion of the cocontinuous phase, and the optimum matrix structure to improve the toughness was obtained on 20 eq % addition of TAIC. For example, when using 20 eq % of TAIC and 5 wt % of PMS (M(w) 303,000), the fracture toughness (K-IC) for the modified resins increased 100% at a moderate loss of flexural strength and with retention in flexural modulus and the glass transition temperature, compared to those of the unmodified cured Matrimid resin.