For the cure process of a tetrafunctional epoxy resin/poly(ether sulfone)/dicyandiamide, we established new isoconversion and isoviscosity TTT(time-temperature-transformation) cure formulae. The transformation here involves isoconversion and isoviscosity contours besides gelation, vitrification and phase separation. The conversion was characterized by d.s.c., the viscosity was by viscoelastic measurement and the phase separation process was by light scattering and scanning electron microscopy. The phase separation took place at lower conversions and corresponding lower viscosities. Two-phase structure is established via spinodal decomposition mode and the phase-separated structure is nearly fixed by a gelation. The gelation occured at a conversion of ca. 45-50% and a viscosity of 10⁴poise over the entire cure temperature. The periodic distance, Am, in the phase-separated structure changed; the lower the cure temperature, the shorter the periodic distance. When the cure temperature was high, a bicontinuous two-phase structure was obtained. For a low temperature cure system, the spherical domain structure is realized.