Abstract: The causes for fracture of H13 large size mandrel are found out by observing the macromorphology of the fracture of a large size mandrel of a Φ460 mm PQF mandrel mill and analyzing the mandrel's mechanical properties and microstructure.The analysis indicates that there are quite a lot of liquation carbides on different radial positions.The liquation carbides are generally in strip shape and some of the aggregated ones are distributed in chain structure, so the toughness of the material is lowered and causes the mandrel to fracture.