Abstract: An industrial experiment was carried out to study the evolution of billet surface porosity in hot rolling of round steel.Holes were drilled at the head of a billet. Linear cracks were spot at the surface of rolled round steel in parallel. Slight decarburization appears around these cracks,which can be seen from the metallograph. Crack measurement results show that the crack depth is mainly influenced by the hole depth. When the hole depth exceeds 4 mm,serious cracks will be generated on the round steel. The crack length is mainly dependent on the hole diameter. When the hole diameter ranges from 1 to 2.5 mm,the crack length limits to 2 to 8 cm.