The effect of small amounts of curing catalysts on the nanostructures and mechanical properties of epoxy/acrylic block copolymer blends were investigated. Poly(methyl methacrylate)-b-poly(n-butyl acrylate)-b-poly (methyl methacrylate) (PMMA-b-PnBA-b-PMMA) triblock copolymer (acrylic BCP) was blended with diglycidyl ether of bisphenol-A epoxy thermosets. The blends were cured using phenol novolac (PN) with curing catalysts at less than 1 wt%. Three different curing catalysts, triphenylphosphine (TPP), 2,4,6-tris(dimethyl amino methyl)phenol (DMP), and 1,2-dimethylimidazole (DMIz), were compared. A polyol-type linkage structure was formed in the cured epoxy matrix using TPP, whereas both polyether-type linkage structure and polyol-type linkage structure were formed when DMP and DMIz were used. As a result, different nanostructures were formed in response to the different linkage structures in the epoxy matrix originated from the same blend composition of epoxy/acrylic BCP/PN. The TPP catalyst gave a spherical nanostructure, while the DMP and DMIz catalysts gave branched cylindrical nanostructures in the epoxy/acrylic BCP/PN blends. The compatibility of the PMMA-block chains of the acrylic BCPs to the cured epoxy matrix was a key factor that determined the nanostructures. The fracture toughness of the cured blends was dependent on both the nanostructures in the blends and the ductility (extent of cross-linking density) of the epoxy matrix.