The influences of inertia, elastic-plastic effect and initial void size are studied on the damage evolution in spallation of ductile metals under planar impact loading by using the hollow spheroid model. Numerical analysis indicates that inertia, elastic-plastic effect and initial void size each have a significant influence. Influence of Initial damage or initial void size on spallation damage cannot be neglected. In this paper we attempt to develop a method to analyse initial void size due to its importance on spallation damage.