Poly(styrene-co-acrylonitrile) (SAN) was found to be miscible with the tetraglycidylether of 4,4'-diaminodiphenylmethane (TGDDM), as shown by the existence of a single glass transition temperature (T-g) over the whole composition range. However, SAN was found to be immiscible with the 4,4'-diaminodiphenylmethane (DDM)-cured TGDDM. Dynamic mechanical analysis (DMA) shows that the DDM-cured TGDDM/SAN blends have two T(g)s. A scanning electron microscopy (SEM) study revealed that all the DDM-cured TGDDM/SAN blends have a two-phase structure. The fracture toughness KIC of the blends increased with SAN content and showed a maximum at 10 wt% SAN content, followed by a dramatic decrease for the cured blends containing 15 wt% SAN or more. The SEM investigation of the KIC fracture surfaces indicated that the toughening effect of the SAN-modified epoxy resin was greatly dependent on the morphological structures.