Armor steel is a special grade of steel that is commonly designed for providing protection of the army resources from different piercing ammunitions. This steel must be assessed through its mechanical properties, and its ballistic resistant to confirm their ability to withstand the resistance of armor-piercing ammunition, before being subjected to use. Case study of armor steel sheet 6mm thickness that has been subjected for normal ballistic test EN1522, one projectile passed through the sheet, while the other two projectiles have fired at the surface of the sheet. Then, this research was designed to investigate the root cause of failure of this armor steel sheet 6mm under ordinary ballistic test. Constituent structures of steel have been monitored and estimated by using Optical, Scanning electron microscope, Micro hardness tester, and X-ray diffractometer. In addition, hardness map at the surface of the investigated sheet has been detected. Finally, thermal transformation of this steel has been determined by using Dilatometer. The results refer that the root cause of failure should be attributed to the inhomogeneity in the microstructure among the sheet area, which certainly refers to the poor homogeneity at the heat treatment cycle used at the production site.