The co-cured joining method, which is regarded as an adhesively bonded joining method, is an efficient joining technique because both the curing and joining process for the composite structures can be achieved simultaneously. In this paper, the manufacturing process of the co-cured joint was introduced and specimens of co-cured single and double lap joints of the plate type were fabricated and tested under a tensile load. The failure mechanism of co-cured single and double lap joints was discussed using stress distributions obtained from finite element analysis. Failure criteria of the co-cured single and double lap joints are proposed and verified by comparing the tensile load-bearing capacity calculated by failure criteria with tensile test results. (C) 2003 Elsevier Science B.V. All rights reserved.