Abstract: The influences of surface defects,deformation temperature,deformation amount,microstructure and nonmetallic inclusions on the cracking of 45 hot-rolled round steel during upset forging at room temperature were studied.The results showed that the brittle alumina and plastic sulfide nonmetallic inclusions were the direct cause of the cracking of 45 hot-rolled round steel during upset forging.These inclusions had little impact on the tensile properties,but had a great influence on the compression performance particularly when they lay in the vicinity of drum-shaped deformation zone.Brittle inclusions brought stress concentration and plastic inclusions formed hollows,resulting in generating crack initiation easily between inclusions and matrixe.Primary sheet pearlite and heat stress formation during hot rolling intensified the tendency of inclusion cracking.