The morphology and rheological behave of the polyethersulfone/bismaleimide blends during isothermal curing was investigated by rheological instrument, different scanning calorimetry (DSC), scanning electronic microscopy (SEM) and time resolved light scattering (TRLS). The influences of the PES content, the PES molecular weight and the curing temperatures on the complex viscosity of the blends were discussed in detail, which suggested that the fluctuation of the complex viscosity at the beginning of phase separation was largely dependent on the PES content, while the PES molecular weight mainly influenced the onset of phase separation and had relatively less influence on the evolution of complex viscosity. Moreover, the evolution of complex viscosity of blends with high PES content was relatively less sensitive to the curing temperature. In addition, it was found that the phase morphology as well as the composition of continuous phase played an important role in the rheological behavior of PES modified bismaleimide resin during isothermal curing.