The morphology, dynamic mechanical behavior and fracture behavior of polyetherimide (PEI)/dicyanate semi-interpenetrating polymer networks (semi-IPNs) with a morphology spectrum were analyzed. To obtain the morphology spectrum, we dispersed PEI particles in the precured dicyanate resin containing 300 ppm of zinc stearate catalyst. The semi-IPNs exhibited a morphology spectrum, which consisted of nodular spinodal structure, dualphase morphology, and sea-island type morphology, in the radial direction of each dispersed PEI particle due to the concentration gradient developed by restricted dissolution and diffusion of the PEI particles during the curing process of the dicyanate resin. Analysis of the dynamic mechanical data obtained by the semi-IPNs demonstrated that the transition of the PEI-rich phase was shifted toward higher temperature as well as becoming broader because of the gradient structure. The semi-IPNs with the morphology spectrum showed improved fracture energy of 0.3 kJ/m(2), which was 1.4 times that of the IPNs having sea-island type morphology. It was found that the partially introduced nodular structure played a crucial role in the enhancement of the fracture resistance of the semi-IPNs.