Significant effect of cellulose nanofibers (CNFs) on cure-induced phase separation in dynamically asymmetric system is reported. An epoxy/polysulfone blends with typical layered structure formation was chosen as the polymer matrix, and morphology evolution and rheological behavior of systems with different nano-size fiber loadings upon curing reaction were investigated using optical microscopy and rheological measurement. CNF distributed uniformly in the polymer matrix and had good interaction with polymer chains. Curing reaction of epoxy was promoted by CNF, making the system gel and phase separate earlier. Meanwhile, system viscosity was increased with CNF addition, and the movement of polymer chains and component diffusion were constrained, as a result, the structure evolution process was slowed down. The CNF altered the final morphologies, resulting in refined structures with smaller characteristic length scales or even completely change the morphologies from the layered structures to a bicontinuous structure when the CNF concentration reached to a relatively high level. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019