Abstract: According to the dimensionless depth-of-penetration formula for different targets penetrated by rigid projectiles, the present paper theoretically analyses the resistive force which the target exerts on the projectile during penetration. In particular, the impact velocity threshold, which is applicable for the assumption of constant resistive force, is formulated by impulse analysis. The impact velocity thresholds are calculated for different projectile/target combinations by the proposed formula and the calculated velocity thresholds are in good agreement with the relative numerical results.