The penetration effects of projectiles impacting rocks are of great significance in the underground protective engineering and the weapon warhead designs. Especially，issue of the overload of the projectiles is more essential as the key technology for the warhead designs，but also great help to make a clear definition of the motion laws of the projectile and the interactions between projectiles and the rock targets. In order to study the overload characteristics of projectiles penetrating rocks，rock-penetrating experiments were carried out using projectiles with diameters f 100 mm and f 300 mm，respectively. The overload versus time curve was recorded by means of an overload measurement system. Based on the overload curves，the projectiles¢ overload can be divided into three phases：the overload of rapid increasing as the contact area increases；the overload reducing slowly after the nose enters the targets；and the overload reducing rapidly to zero when the projectile¢s energy is almost used up. From the achieved overload curves，it can be obtained that the projectiles have the peak overloads when the penetration depths come up to two times the diameter of projectile. The rocks around the projectile¢s nose are damaged due to the stress wave of penetration，and then，the targets have less ability to prevent the projectilesmotion. On the basis of the experimental results，a new formula of penetration depth has been presented here with reference to experimental data of other tests. The precision is not lower within certain range in comparison with the widely accepted Young¢s formula. Numerical simulation is in good agreement with the experimental results.