The dynamic mechanical behavior of polyetherimide(PEI)/dicyanate semi-interpenetrating polymer networks (semi-IPNs) was analyzed by using the conventional Takayanagi model for the sea-island morphology (PEI content of 10% or below) and a polyhedron model, which we developed for the dicyanate nodular structure (PEI content of 20% or above). The model parameters for the Takayanagi model were determined by the volume fraction of the PEI dispersed phase and Kerner＇s equation, while the model parameters for the polyhedron model were simply calculated by introducing a phase continuity factor. Both models correlate well with the morphology revealed in the micrographs except near the transition temperature of the PEI-rich phase. The additional shift toward higher temperature found in the nodular structure was due to the intermixed phase composition, and the effect was quantified by measuring the calorimetric transition temperature. The dynamic mechanical modeling for the dual-phase morphology (PEI content of 15%) and for the semi-IPNs showing a morphology spectrum were also successfully accomplished by combining the Takayanagi model and the polyhedron model.