The morphology and fracture behavior of polyimide modified carbon fiber/dicyanate composites were analyzed. Composites having morphology gradient were prepared by inserting a polyimide film between prepregs of carbon fabric impregnated by dicyanate alone, and controlling the relative rate of dissolution of polyimide and the polymerization of dicyanate by adding zinc stearate catalyst. The composites having morphology gradient showed G(IC) and G(IIC) values of 2.2 kJ/m(2) and 1.4 kJ/m(2) respectively, which showed 3-5 times of those values of unmodified carbon fiber/dicyanate composites.