Abstract: The mechanism of cracking in 304L seamless steel pipe during double-roll cross piercing processing was analyzed by optical microscopy, thermal-calculation ulation software and scanning electron microscopy. The results show that about 3 vol.% δ ferrite phase in the billet and the heterogenous deformation of billet during the piercing process were the main causes of cracking. In addition, the difference of deformability between δ ferrite phase and γ phase caused both the tensile stress and the shear stress at the interface of two phases higher than the fracture strength of billet, which promoted the formation and growth of the cracks.