Large vacancy clusters in an aluminum single crystal with a low dislocation density were observed by X-ray Lang camera after slow cooling from high temperature. The clustering process of vacancy was simulated by using a model which takes into account the association and the dissociation of vacancy and the growth to larger clusters. Comparing the simulated results with the experimental one, it is confirmed that impurities or complexes composed of impurities are necessary for the formation of large vacancy clusters as a nucleus. In order to make certain , Iarge vacancy clusters have been examined with a electron microscope.