The ballistic behaviour of a bi-layer ceramic-metal target against steel projectile with varying layer thicknesses has been investigated using a three-dimensional finite element model. The bi-layer target was made of alumina 99.5 % ceramic front layer and aluminium 2024-T3 metallic back layer with an areal dimension of 100×100 mm and the thickness of both layers were varied, with the total thickness of the composite being kept as 10 mm and 20 mm. A steel 4340 cylindrical blunt-nosed projectile was used with 30 grams mass and 10.9 mm diameter. The Johnson-Holmquist 2 (JH-2) constitutive model was used for reproducing the high strain behavior of alumina and Johnson-Cook (JC) model was used for aluminium alloy and steel. The impact velocity of the projectile was varied in the range 200-700 m/s for 10 mm total thickness and 500-800 m/s in the case of 20 mm total thickness for studying the effects of thickness ratios on ballistic resistance of the bi-layer target. The residual velocities were compared and the ratio of front to back layer providing the highest ballistic limit velocity was found for both cases.