Abstract: Based on the investigation of many previously published ballistic tests and the thermoplastic constitutive model of Bai and Johnson, a simplified analytical model is proposed for analysis of energy absorption in plugging failure of thin metallic plates struck at normal and subordnance velocity by rigid projectiles with flat noses. This model takes into account the effects of plate material rupture strain and strain rate on critical impact energy. A new dimensionless semi-empirical formula for critical impact energy and ballistic limit of thin mild steel plates is developed according to the proposed model and previously published ballistic test data. The results of ballistic limit from the formula developed in this paper are in good agreement with those from ballistic tests and generally more accurate than those from previously published empirical formulas.