An experimental program has been conducted to study the perforation of stiffened steel plates by truncated oval-nose projectiles at normal impacts. The projectiles of 104mm diameter were fired at 921A stiffened plates with thickness 15.mm for the base plate at an initial velocity from 560m/s to 620m/s. High speed camera system was employed to record the perforation process and from the digital images, impact angles and projectile velocities during perforation were determined using image processing. Numerical simulations have been performed for projectiles against single and layered plates adopted in the experiments. The perforation process is explored in details and the deformation modes are obtained. By adopting an energy method, a four-stage analytical model that accurately predicts the residual velocity is developed. Simulation results, experimental data and theoretical predictions are reasonably well correlated.