In this work, the rheological behavior and structural transitions in poly(ether sulfone) (PES)-modified epoxy systems during phase separation were studied by rheometry instrument, scanning electronic microscopy, time-resolved light scattering, and differential scanning calorimetry. The molecular weight and concentration of PES have profound effects on rheological behavior of modified epoxy systems. It was found that the evolution of complex viscosity is closely connected to the molecular weight of PES. Blends with higher molecular weight PIES tend to have a lower conversion at the onset of phase separation and show quicker increase of viscosity with Curing process. The complex viscosity exhibits an exponential growing process during phase separation at various temperatures. The relaxation time of rheological growth depends on both molecular weight of PES and test frequency.