The effects of incorporating a thin layer of various types of polystyrene-poly(methyl methacrylate) copolymers, i.e. block, random, and graft copolymer on the interfacial adhesion between PS and PMMA have been investigated. The fracture toughness of the interface was measured using an asymmetric double cantilever beam fracture test and the enhanced toughness effect of copolymers was compared. The fracture toughness of the interface increased with increasing layer thickness of the copolymer and each copolymer had the layer thickness of saturated fracture toughness. The fracture toughness increased in the order of block > graft > random copolymer.