Abstract: The HRB400 hot rolled ribbed steel bars produced by a company fractured during cold bending. The causes of fracture were analyzed by means of macro fracture analysis, chemical composition analysis, metallographic examination, scanning electron microscopy analysis, energy spectrum analysis and so on. The results show that due to the serious chemical composition segregation of the HRB400 hot rolled ribbed steel bars, the microstructure of the matrix was inhomogeneous, and there were abnormal microstructures such as martensite and bainite, which resulted in high strength and poor plasticity of the steel bars. In addition, a large number of nonmetallic inclusions existed in the steel bars, which destroyed the continuity of the steel bars matrix and made the steel bars matrix easily produre stress concentration and form voids or cracks. These two factors eventually caused the cold bending fracture of the steel bars.