A simulation tracing study has been performed for the effects of different cooling rates on the evolution of microstructures during solidification process of liquid metal Ga by means of molecular dynamics method. The bond-types and basic cluster structures of metal Ga have been analyzed by using HA bond-type index method and cluster-type index method. It has been found that the cooling rate plays an important role to the microstructures in the solidification processes. At the cooling rates of 1.0×1014 K/s, 1.0×1013 K/s and 1.0×1012 K/s, the amorphous structures would be formed in the system with the rhombohedron related to 1311, 1301 bond-types as the main body, and coexist with the fcc, hcp and other structures; at the cooling rate of 1.0×1011K/s, crystallization would take place in the system, the crystalline transition temperature Tc being about 198K, and the lower the cooling rate, the higher the Tc. The crystal would have the orthorhombic structure related to 1421 bond-type(determined by visualized method) as the main body. This gives us a new way for researching the crystalline transition processes of liquid metals.