Abstract: One X80 steel oil pipe experienced longitudinal cracking during JCOE forming, and the crack length was getting up to 700 mm. The cracking reason was analyzed by means of chemical composition analysis, mechanical properties testing, fracture analysis and metallographic examination. The results showed that the primal crack resulted in pipe cracking during forming, and steel plate near the crack possessed low fracture toughness, which accelerated crack propagating under tension stress of forming. The existence of non-metal inclusions of big size was responsible for the generation of primal crack.