Abstract: A 45 steel cap nut was found to have surface crack during assembly. After subsequent screening, 116 of the 1 069 cap nuts delivered in the same batch had the same surface defects as the failure one. In order to find out the cracking reason, macro and micro observation on crack and fracture, energy spectrum analysis, and metallurgical structure examination were carried out. Based on these experimental results and production process inspection result, the final cracking reason of cap nut was determined. The results show that fold defects existed in raw material, which is the main reason for cracking. The primary fold defects became cracking sources when quenching, and extended in the thickness direction under the stress of quenching. It is proposed that nondestructive inspection should be used and process discipline management should be strict in order to avoid and prevent such failure.