A co-cured joining method has several advantages over an adhesively-bonded joining method for its simple manufacturing process. It requires neither an adhesive nor surface treatment of the composite adherend and uses excess resin to bond both adherends, including composite adherend. In this paper, the effects of bond parameters, including surface roughness on the steel adherend, stacking sequence of the composite adherend, and manufacturing pressure in the autoclave during the bonding process, on tensile load bearing capacities of plate-type co-cured single and double lap joints were investigated experimentally. Bond parameters were related to interfacial parameters such as a contact area and adhesive thickness, which affected the joint strength. Experimental results were explained with respect to all of interfacial parameters and bond parameters through investigating failure surfaces of the steel adherend and a lot of data of tensile load bearing capacities. In order to predict tensile load bearing capacities of co-cured single and double lap joints, we considered two different failure models using stress distributions in both co-cured lap joints obtained from finite element analysis. Finally, optimal conditions to improve the joint strength of co-cured single and double lap joints have been presented.