A three-component bismaleimide resin, composed of 4,4 ' -bismaleimido-diphenyl methane (BDM), o,o ' -diallyl,bisphenol A (DBA), and o,o ' -dimethallyl bisphenol A (1.0/0.3/0.7 eq ratio) was used as a parent bismaleimide resin. Modification of the three-component bismaleimide resin was examined by blending it with poly(ether ketone ketone)s. Poly(ether ketone ketone)s include poly(phthaloyl diphenyl ether) (PPDE), poly(phthaloyl diphenyl ether-co-isophthaloyl diphenyl ether) (PPIDE), and poly(phthaloyl diphenyl ether-co-terephthaloyl diphenyl ether) (PPTDE). The PPIDE (51 mol % isophthaloyl) and PPTDE (44 mol % terephthaloyl) were more effective as modifiers for the bismaleimide resin than was PPDE. For example, the fracture toughness (K-IC) for the modified resin increased 30% with no deterioration in the flexural strength and modulus with a 15 wt % inclusion of PPTDE (MW 23,400) compared to the parent three-component bismaleimide resin: the K,, increased 95% compared to the value for the Matrimid 5292 resin composed of BDM and DBA. The morphologies of the modified resins changed from particulate to cocontinuous phase structures, depending on the modifier structure and concentration. Toughening of the cured bismaleimide resin could be achieved because of the cocontinuous phase structure.