The interfacial reinforcement between polypropylene (PP) and amorphous polyamide (aPA) has been investigated by in-situ reactive compatibilization. PP reactivity with aPA was introduced by melt blending a certain amount of maleic anhydride grafted polypropylene (mPP) with pure PP. The effects of the mPP content, bonding time, and temperature on the interfacial fracture toughness, G(C), of the aPA/(PP + mPP) adhesive joint were studied. The G(C) values were measured by means of a wedge test in an asymmetric double cantilever beam geometry. The G(C) enhancement was proved to be due to the in-situ formation of copolymers at the interface by the use of contact angle measurements and X-ray photoelectron spectroscopy (XPS) analysis. Incorporation of 3 wt % mPP in the mixed PP would be enough for effective interfacial reinforcement in this particular system. At a certain bonding temperature, the interfacial fracture toughness reaches the saturation value G(C)* above the critical bonding time. The value of G(C)* increases, but not monotonically, with critical areal density of copolymers at the interface Sigma(C) as a result of increased bonding temperature. The effect of bonding temperature on the G(C)* can be divided into three distinct temperature regions : (i) a region below the glass transition temperature of aPA and the beginning temperature of melting of mixed PP where there is virtually no adhesion; (ii) a region across the melting range of mixed PP crystals where the interfacial fracture toughness steadily increases with temperature; (iii) a region well above the melting temperature of mixed PP where G(C) becomes relatively low. Scanning electron microscopy (SEM) and XPS results show that fibrillation of PP induced by the interfacial adhesion and subsequent breakage of the fibrils are the characteristics of fracture mechanism. Due to different crystalline morphologies of PP at different bonding temperatures, fracture mechanisms phenomenologically vary with bonding temperature at the microscale. Analysis on the locus of failure by microscopy and XPS reveals that the interfacial fracture toughness G(C) of amorphous and semicrystalline polymer pairs in the reactive system is influenced not only by the areal density of copolymers formed at the interface but also by the crystalline morphology of the system.