We studied enhanced interfacial adhesion between a flexible amorphous polymer (polystyrene, PS) and a semi-crystalline polymer (a polyamide, Ny6). The surface modification of PS to provide functional groups reacting with the functional groups of Ny6 was carried out with ion-beam and/or plasma treatment. These surface modifications were found to significantly alter the interfacial adhesion strength between PS and Ny6. Though the fracture toughness was varied depending on the process, its overall behavior was quite similar with one another; the fracture toughness increased with increasing bonding temperature and bonding time, passed through a peak, and then decreased with further increase of the bonding time or temperature.