Poly(styrene-co-acrylonitrile) (SAN) with 25 wt% acrylonitrile (AN) content was found to be miscible with uncured bisphenol-A-type resin, i.e. diglycidylether of bisphenol A (DGEBA), as shown by the existence of a single glass transition temperature within the whole composition range. Miscibility between SAN and DGEBA is considered to be due mainly to entropy contribution. However, SAN was judged to be immiscible with the 4,4’-diaminodiphenylmethane-cured epoxy resin (DDM-cured ER) as revealed by the means of differential scanning calorimetry (d.s.c), dynamic mechanical analysis (d.m.a.) and scanning electron microscopy (SEM). It was observed that the DDM-cured ER/SAN blends have two T-g’s, which remain almost invariant with composition and are close to those of the pure components, respectively. SEM study revealed that all the DDM-cured ER/SAN blends have a two-phase structure. The fracture mechanics studies indicate that the DDM-cured ER/SAN blends containing 10 wt% give a substantial improvement of fracture toughness K-IC. The fracture toughness K-IC increases with SAN content and shows a maximum at 10 wt% SAN content, followed by a dramatic decrease in K-IC for the cured blends containing 15 wt% SAN or more. SEM investigation of the K-IC fracture surfaces indicates that the toughening effect of the SAN-modified epoxy resin is greatly dependent on the morphological structures.