To understand the e ects of substrate materials on the fracture behavior of adhesive joints, experimental studies and รฟnite element analyses have both been conducted for double-cantilever-beams (DCB) with aluminum and steel substrates at di erent bond thickness (h). Numerical results show that the region dominated by the crack singularity is much smaller than by the bond thickness. Very small plastic deformation may hence violate the requirements for small-scale yielding where the crack-tip รฟeld can be characterized uniquely by the stress intensity factor. Both critical strain energy release rate and J -integral for the joints with steel substrate are lower than those for the joints with aluminum substrate. Compared to the critical strain energy release rate, the critical J -integral is less sensitive to the substrate material if small plastic deformation occurs before cohesive failure takes place through the adhesive layer. For the joints with aluminum substrate, the fracture toughness initially increases and then decreases with bond thickness. Elastic-plastic crack-tip analysis indicates that at the same level of loading, a higher opening stress is observed in the joint with a smaller bond thickness. A self-similar stress รฟeld can be obtained by the normalised loading parameter, J=h 0 .