The conventional studies on the granular material impact dampers, rarely take the dependency of restitution coefficient on relative impact velocity due to elasto-plastic deformation at the impact into consideration. In this paper, elasto-plastic materials such as lead are treated. The elasto-plastic behavior at the impact is modeled as a bi-linear type force-deformation characteristics. Impact behaviors in the horizontal and vertical plane are evaluated by numerical simulations, and the viblation mitigation capability is investigated.