The joints are often the weakest areas in composite structures. In this paper, the thick aluminum-aluminum joint specimens and thick composite-aluminum single lap adhesive joint specimens were manufactured and the tensile tests were performed. The fracture mode of each specimen was investigated and the modified damage zone theory based on the yield strain was proposed and compared with experimental failure load of each mode. The failure loads of the thick aluminum-aluminum joint and composite-aluminum joint were predicted by the same failure criterion and they could be predicted to within 19.3% using the damage zone ratio method for all 14 cases investigated.