Abstract: It ia difficult to coparate carboxyl-randomized butadiene acrylonitrile copolymer(CBN) from phenolic epoxy resin(F-51)in au anhydride curing system because of the food compatibility of CBN/F-51.With the increasing of the amount of CBN in the resin,the glass transition peak of the blend is widened greatly for the semi-compatible microstructure of the dual phase continuity.It is found that the phase separation of CBN from the resin is promoted evidently by adding small amount of butadiene acrylonitrile copolymer (BN-26) into the CBN/F-51 blend system.