Abstract: In order to analyze the cause of abnormal tensile fracture of small specification 40Cr round steel, the microstructure, micro-hardness, fracture morphology, inclusions, carbides were observed by metallographic microscope, micro-hardness meter, scanning electron microscope and energy dispersive spectrometer. The results show that the abnormal fracture is mainly caused by the segregation of carbon, alloying elements and phosphorus in the steel.