The morphology, tensile properties, and fracture behavior of polyetherimide (PEI)/dicyanate semi-interpenetrating polymer networks (semi-IPNs) with a morphology spectrum were analyzed. Semi-IPNs with a morphology spectrum were prepared by inserting PEI film into neat dicyanate resin, by controlling the relative rate of the dissolution and diffusion of the inserted PEI film, and by controlling the curing reaction of the dicyanate resin using a zinc stearate catalyst. The morphology spectrum was obtained by the accompanied phase separation during curing. Final morphology of the cured dicyanate revealed three types of morphology along thickness direction because of the PEI concentration gradient formed: (1) nodular spinodal structure where the concentration of PEI was 18 wt % or more; (2) dual-phase morphology having both sea-island region and nodular structure where PEI concentration was about 15 wt %; (3) sea-island morphology where PEI concentration was 12 wt % or less. Semi-IPNs with the morphology spectrum displayed improved tensile properties in IPNs with the overall PEI content less than 15 wt %. In addition, semi-IPNs with 7.5 wt % PEI having the morphology spectrum showed a critical strain energy release rate, G(IC), of 0.61 kJ/m(2), which was 2.1 times that of the IPNs having uniform single morphology (sea-island structure). It was found that the tensile strength and the fracture toughness enhancement were proportional to the layer thickness having nodular structure and/or dual-phase morphology.