Abstract: A simulation study on the formation characteristics of clusters in a large-scale liquid Al system consisting of 105 atoms has been performed by the molecular dynamics method. And a cluster-type index method（CTIM）has been used to describe the structural configurations of various clusters. The results demonstrate that the icosahedron clusters（12 0 12 0）and their combinations play the most important role in the microstructure transition. The nanoclusters（containing up to 104 atoms）have been formed by combining some middle clusters which have been formed by combining smaller basic clusters. The structures of these nano-clusters are very different from those of 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. The center atoms of these basic clusters are bond-connected each other with the linear or twisting mode. The corners of the nano-cluster just could become the starting points of the dendrite growth in the solidification processes of liquid metals.