In this paper, stress distributions in a co-cured single lap joint subjected to a tensile load were investigated using the finite element analysis. Residual thermal stresses, which resulted from the curing process of the co-cured single lap joint, were also considered. Since the adhesive layer in the co-cured single lap joint was about 10 mum thick, very thin compared with the thickness of both adherends, the interface between the steel and composite adherends was assumed to be perfectly bonded. The co-cured single lap joint was analyzed with respect to several bond parameters such as the bond length and stacking sequence of the composite adherend. The failure mechanism of the co-cured single lap joint was partial cohesive failure in the composite material, which was significantly affected by the interfacial tensile stress at the free edge of the co-cured single lap joint. Interfacial tensile stress was a primary factor that caused interfacial delamination between the steel and composite adherends in the co-cured single lap joint. Finally, tensile load-bearing capacities calculated from the Ye-delamination failure criterion were compared with the experimental results, and relatively good agreement was found.