The reaction-induced phase separation process of polyethersulfone- and polyetherimide-modified (PES- and PEI-modified) epoxy systems was investigated by using transmission electronic microscopes (TEM) and a rheometry instrument. The morphologies of cured blends alter considerably with the PES content and molecular weight at the same content of total thermoplastics. Compared with the PEI-modified systems, many complex phase separation mechanisms are observed due to the introduction of PES, but similar relationships between melt viscosity change behaviors and morphology developments are observed in the PES-added ternary systems. Dynamic mechanical analysis (dma) and mechanical studies show that neither heat resistance nor toughness of the resultant blends is obviously influenced by the addition of PES.