Abstract: Through process comparison test, chemical composition analysis, metallographic analysis, scanning electron microscopy and energy spectrum analysis, ultrasonic detection analysis and other methods, the causes of inner wall scar defect produced during piercing of GCr15 bearing steel pipes were analyzed. The results show that the scar defect on inner wall of the pipes was caused by the serious center porosity defect of the bearing steel bars which was torn and extended during piercing. The quality of the bearing steel bars can be controlled by ultrasonic flaw detection to avoid the occurence of similar defects.