Abstract: Part of the SWRCH18A screws cracked at heads in the pier process from cold heading steel wire rod to screws. Macro-observation, metallographic examination, non-metallic inclusion test and electron microscopy examination, as well as micro hardness test on defective parts and EDS analysis were carried out to find out the failure cause of the screws. The results show that the main failure cause is that dissimilar mental inclusion existed near the surface of the internal wire rod. The dissimilar mental inclusion was mainly caused by improper melting operation and extra silicon-manganese alloys, which could not be incompletely molten or unevenly spread. The dissimilar mental inclusion is not allowable flaw, which destroys the integrity of steel microstructure and ruptures in the process of subsequent rolling, drawing and cold heading.