Abstract: The inclusion and metallographic structure of defect sample of φ8mm spiral steel HRB400E were studied by optical metallography, and the gas element contents were measured by nitrogen-oxygen-hydrogen analyzer. The results show that the content of O element in the defect is high, and there are a lot of silicate inclusions and spherical oxide inclusions with grade of C3.0s and D3.0 at the cross section of the defect. The inclusions cause the billet to crack in the continuous casting process and it expands to the surface of the steel bar during the rolling process, resulting in the "rotten steel" defect. The reasons of the inclusion are high oxygen content in molten steel at the end of converter, insufficient argon blowing time in ladle furnace,and slag entrapment in continuous casting ladle.