Based on the unbounded matrix, a void growth model is proposed in this paper. In this mod el, extending the Griffith criterion of energy conservation during the expanding of cracks to the case of impact loading, we propose an assumption that the strain energy in matrix can transform into its surface energy and kinetic energy during the micro-void growth. The void growth velocities of the OFHC copper and LY12 alu minium are calculated, and the result of which agrees with that of Seaman.