The hot-rolled ribbed steel bars of reinforced concrete occurs fracture when being tested. By using chemical analysis, metallographic examination, mechanical properties testing, manufacturing process analysis and other methods, this paper analyzed and looked up the causes of producing cold-formed fracture. The results showed that： In the continuous casting, the protective slag penetrated, resulting in partial carbon ：content increased, so that partial changes in mechanical properties is the main reason leading to cold-bended fracture.