Abstract: The shining lines like cracks with metallic luster were found on pipe wall of 45Mn2 steel tube when lathe machining. Macro and micro structure inspection, mechanical properties testing, flatten testing and corrosion testing methods were used to analyze the formation cause of shining line defect. The results show that the microstructure of the shining line was bainite and it was line of chemical compositions segregation, which came from the continuously cast billet. The shining line was defect that should not exist. It was recommended that steel mill took measures to improve the quality of billets and put an end to the formation of shining line defect.