Abstract: DH36 steel pipe for offshore platform cracked during expanding diameter. The cracking causes of the steel pipe were analyzed by means of chemical composition analysis, scanning electron microscope analysis, energy spectrum analysis, oxidation analysis and metallographic examination. The results show that the cracking of DH36 steel pipe was caused by the original defects of steel plate. When the billet was heated in the heating furnace, the surface of the defect will oxidize again to form oxidation dots. During rolling, the defect will be rolled and deformed to form cracks. When the steel pipe was expanded diameter, the cracks growth eventually led to the cracking of the steel pipe.