Abstract: The numerical simulation of the penetration of a spherical-nosed 4340 Steel Long Rod into Semi-infinite 6061-T6511 Aluminum Targets is performed with ALE method and Steinberg’s constitutive model using the ANSYS/LS-DYNA finite element code. It transpires that the state of the steel long rod penetrator changes with increasing impact velocity: first it penetrates the aluminum alloy targets as a rigid body, then as a deformable body without mass loss and finally as an erosive body at higher impact velocities, which is in agreement with the experimental observations made by Forrestal et al. It also transpires that for a deforming non-erosive penetrator the head of the penetrator becomes bigger only in the initial phase and followed by the subsequent thickening of the shank during which the velocity of the penetrator tail decreases rapidly whilst the penetration velocity remains relatively steady; that the stresses in the penetration direction at the two ends of the transition zone between the deformed region and undeformed region are close to Hugoinot Elastic Limit and its initial yield stress, respectively.