The effect of molecular weight between crosslinks, M-c, on the fracture behavior of rubber-toughened epoxy adhesives was investigated and compared with the behavior of the bulk resins. In the liquid rubber-toughened bulk system, fracture energy increased with increasing M-c. However, in the liquid rubber-toughened adhesive system, with increasing M-c, the locus of joint fracture had a transition from cohesive failure, break in the bond layer, to interfacial failure, rupture of the bond layer from the surface of the substrate. Specimens fractured by cohesive failure exhibited larger fracture energies than those by interfacial failure. The occurrence of transition from cohesive to interfacial failure seemed to be caused by the increase in the ductility of matrix, the mismatch of elastic constant, and the agglomeration of rubber particles at the metal/epoxy interface. When core-shell rubber, which did not agglomerate at the interface, was used as a toughening agent, fracture energy increased with M-c.