Abstract: Circuit defect is the most fundamental and difficult problem in the process of linkage synthesis. A Stephenson six-bar chain contains a basic four-bar loop and a dyad. Based on the distance of two common joints shared by the basic four-bar loop and the dyad,the principle is elucidated and the model is established for circuit identification of Stephenson six-bar chains in the paper. The criteria and procedures to automatically identify the circuit and circuit defect of Stephenson six-bar chains are developed. The results provide theoretical basis and technical method for computer aided dimensional scheme selection of Stephenson six-bar mechanisms synthesis. In addition, the model for circuit identification can be directly used to solve the crank and motion order problem that are often encountered in Stephenson six-bar mechanisms synthesis.