Abstract: Based on an investigation on elements reponsible for the cracks of 18Cr-2Mo ferrite stainless steel shell and cold-rolled pipe, a conclusion is made that it is the double weakening effects by comparatively high contents of C,N,S interstitial atoms and their flake deposition on the 100\ face that caused the cracks. Therefore, to reduce the amount of the said atoms will be effective on curbing development of the said cracks.