The effect of surface functionalization on the interfacial adhesion between two immiscible semicrystalline polymers, polypropylene (PP) and a polyamide (nylon 6, Ny6)), was investigated. The surface of PP was functionalized by a low-energy ion-beam-assisted gas reaction. Surface functional groups containing carboxyl and carbonyl groups created remarkably different interactions at the interface. Fracture toughness was measured using an asymmetric double cantilever beam test (ADCB). The calculated fracture toughness was significantly increased for the functionalized PP case. The system with reactive oxygen gas added showed a higher fracture toughness than the case with only argon-ion-beam irradiation because of more reactions and/or interactions between the functionalized PP surface and Ny6. Analysis on the locus of failure by using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) revealed that the fracture toughness between PP and Ny6 was influenced not only by the bonding temperature but also by the bonding time at constant bonding temperature. The fracture toughness increased after some induction time of annealing; then, it reached a plateau value. The fracture toughness increased with the bonding temperature, showed a maximum at 200 degreesC, and then decreased at a higher temperature of 210 degreesC. This behavior is different from other reported results. The present system is more or less like a chain tethered to a solid surface. The failure was caused by the weaker of the adhesive strength or the cohesive strength. The effect of bonding temperature is attributed to the cohesive failure in the PP phase. The adhesive strength increased with the bonding temperature while the cohesive strength decreased with the bonding temperature because of less entanglement of reacted chains with the other chains in the bulk. The dependence of the fracture toughness on the bonding time was explained in terms of this fracture mechanism.