Abstract: Analyzed here in the essay is the cause for the crack of the 25Cr super dual-phase stainless steel bar as occurring during the extrusion process. Based on the thermodynamic equilibrium diagram,the origin of the intermetallic compound σ phase is analyzed. Accordingly,it is concluded that it is just the said σ phase that acts as the main cause for occurrence of the above mentioned crack. Therefore the steel bar is subjected to solid solution again,and the extrusion process is optimized so as to reduce the σ phase in the matrix. Again the extrusion test is conducted. As a result,the tube after the extrusion presents no crack and has good surface quality,being satisfactory to the requirement for the subsequent cold-rolling process.