A simulation study on the formation properties of nano clusters in a large scale liquid Al system consisting of 400,000 atoms has been performed by a molecular dynamics method. And a cluster type index method (CTIM) has been also used to describe the structural configurations of various clusters. The results demonstrate that the icosahedron clusters (12 0 12 0) and their combines play the most important role in the microstructure transition; the nano clusters (containing up to 150 atoms) are formed by combining of some middle clusters which also are formed by combining smaller clusters; the structures of these nano clusters are very different from that of those nano clusters obtained by evaporation, ionic spray methods, and so on, the latter is formed by the multi shell crystals accumulated with an atom as the center and the surrounding atoms arranged according to octahedron configuration. These results just can be used to explain the essential distinction between the cluster configurations of Al formed by different ways.