Abstract��Core cracks were detected in FAS2271 hot rolled steel round bars during ultrasonic testing. Samples were then taken from non-conformance steel bars�� and analyzed by metallographic microscope. The results showed that the core cracks of FAS2271 hot rolled steel round bars were mainly caused by non equalization heating during the heating of billets�� and meanwhile�� because the cooling speed was fast after rolling�� and steel bars were directly straightened�� strain overlaid�� the risk of core cracking was increased. By improving the processes of billets heating and slowing the cooling process of steel bars after rolling�� which included increasing the heating temperature�� controlling the pulling speed of billets and adding the process of pit cooling after rolling�� the nonconformance ratio of ultrasonic testing was lowered obviously from 14 percent to 0.15 percent.