The evolution of preexisting nano-voids of different size in single crystal copper is investigated by means of molecular dynamics (MD) simulations. The results show that void collapse depends upon the size of the void. Numerous dislocation loops emerge for large void and phase transformation takes place in the sample. When the shock wave passes the void from left to right，dislocation loop emerges in the right of the void first for small void. However，loops emerge in the right and the left at the same time when the size of the void increases to a critical size，which is decided by the distribution of the stress near the void. Once dislocation loop emerges and grows，the frontal dislocation loop in the right of void grows faster than that in the left for the same void. When the size of the void increases，the velocity of frontal dislocation loops is essentially unvaried，and the angle between the direction leading from the centre of the void to the initial position of collapse and the direction of impact increases.