In this paper, preloading on plates and obliquity in impacts are brought together in order to achieve more realistic conditions for the plates such as in aerospace applications. A numerical model was established to simulate the oblique impact on preloaded aluminum plates. In the design of impacts, numerical simulations were carried out using three variable parameters namely preloading condition, oblique angle and impact velocity. Based on the results, preloading leads to crack initiations on the plates due to sharp edges around the perforation holes. On the other hand, full perforation is observed at low oblique angles however, deformation mode turns to tearing by increasing obliquity of the plates. Impact velocity is another important factor on the impact resistance of the plates. Despite high oblique angles, the plates can be perforated under high-velocity impacts.