The enhancement of the interfacial adhesion between immiscible amorphous polyamids and polystyrene(PS) by the addition of a model reactive compatibilizer was investigated. The model reactive compatibilizer, which were maleic anhydride(MAH) terminated PS with two different molecular weights(60K and 100k), were prepared by anionic polymerization. Fracture toughness of the interface was measured by employing the asymmetric fracture test. Under the annealing condition of 190℃ for 2 hrs the amount of the reactive compatibilizer required to see any adhesion improvement was increased to large extent and the toughness value remained low compared to the commercially available styren-maleic anhydride random copolymer. This is due to the small amount of the functional groups(HAM) which are able to react with amine end-groups of the amorphous polyamide to from in-situ copolymers in case of the end-functionalized PS(ef-PS). It was noted that smaller amount of the 60K ef-PS was required to inprove the interfacial adhesion when compared with the 100K ef-PS. When the amount(thickness) of the ef-PS was replotted with thickness/molecular weight, which is proportional to the amount of the functional group, the two different molecular at the interface is a crucial factor. The difference in the plateau value of the fracture toughness for two different molecular weight ef-PS was interpreted in terms of either the molecular weight dependence of the fracture energy or the residence time of the ef-PS at the interface during diffusion.