Abstract: Based on the results of experimental penetration of small diameter, it is shown that metallographic analysis can supply much detailed information about metallurgical events that happen during the penetration of metallic projectiles through targets of 45 # steel. The actual process of a projectile penetrating through an armour plate is a very complicated physical procedure. Microstructures of the penetrated hole in the 45 # steel plate are studied using SEM and optical metallography. From the penetrated surface to the interior of the steel plate, the section can be divided into four layers: the melted and rapid solidification layer, the recrystallized fine grain layer, the deformed fine grain layer and the normal matrix. In the fine grain layer and the deformed layer, deformation of ferrites grains appears far more intense than that of pearlites, and some pearlites split, with micro-cracks and micro-pores forming around the pearlites. The microstructures of the residual projectiles are also investigated and its consumption of mass is limited within a narrow area of 3～4 mm near the tip.