A tracing simulation study has been performed for the solidification process of liquid metal Ni with four different cooling rates by means of molecular dynamics method. The pair distribution function g(r) curves, the bond-type index method of Honeycutt-Andersen (HA) and the cluster-type index method have been used to analyze the variations of microstructures during the solidification processes. The results show that the cooling rate plays a critical role in the transitions of microstructures. When the cooling rates are 1.0×1014 K/s and 1.0×1013 K/s, the amorphous structures are formed mainly with the 1551 and 1541 bond-types or polyhedron basic cluster (13 １ 10 2) and icosahedron cluster (12 0 12 0) in the system. When the cooling rates are 1.0×1012 K/s and 1.0×1011 K/s, the crystal structures are formed mainly with 1441 and 1661 bond-types or the bcc basic cluster (14 6 0 8) in the system. At the same time, it has been found that the effects of different cooing rates on the microstructures of metal Na are very small in liquid and supercooled states, however, they are very remarkable in solid (amorphous and crystal) states, and can be fully displayed only near the liquid-solid transition points,i.e., the glass transition temperature Tg and the crystallization temperature Tc, respectively. Accordingly,it possibly provides a new method to determine the Tg and Tc of liquid metals.The cluster-type index method would be more favorable than the bond-type index method for investigating the concrete structural characteristics of the disordering liquid, the amorphous and some crystallized systems.