Abstract: Both inner and outer folds are the common defects of the seamless steel tube as developed during the manufacturing process.And formation of such defects are remarkably affected by the inclusions in the billet for pipe-making.With the FEA method, the stress/strain distribution of the 16Mn steel containing stripe MnS inclusion which is under load are analyzed to identify the influence by the inclusion on the inner fold of the pipe.The analysis reveals that stress and strain concentrations exist on the boundary between the MnS inclusion and the base metal, and the combining power by the inclusion with the base metal is rather weak, which leads to the nucleation of the crack at such a location.The larger the size of the crack is, the more serious the stress/strain concentration will be, and the faster the crack extends, the more possible for it to get to the critical size for unstability to form the defective inner fold.Since at the end of the stripe inclusion, there exists rather high stress/strain concentration, development of the stripe inclusion should be strictly controlled.