In the collision between the flat surface of solid and of a sphere, it can be considered that a high stress will be occur, since an impact load is applied in a small local contact area. If an impact stress exceeds the yield stress of a material, the local deformation of the contact surface converts from elastic to plastic flow. Therefore, the impact load may be moderated. It is important when formulating the impact load that the effect of the local plastic deformation of the contact surface on the time history of a load due to impact is investigated. On the other hand, it is revealed by Hertz's theory that the relationship between a force and the amount of approach of both solids during impact is nonlinear. If it is postulated as being linear, there is not little what be able to apply it to analyze the dynamic impact problem. In this paper, the effect of the local displacement behavior in the contact process on the impact load is shown by the experiment of a collision between a plate of mild steel and a high hardness steel ball. The impact load is evaluated by a piezo electric plastic transducer. The limitation which the modified approach of Hertz's theory is valid, are distinguished.