Molecular dynamics simulation study has been performed on the effects of thermal history of initial melt temperature on microstructures during solidification of liquid metal Na. The pair distribution function g(r) curves, the bond-type index method and the cluster-type index method have been used to analyze the variations of microstructures during the solidification process. The results show that the solidification structures of liquid metal Na at the cooling rate of 1×1011 K/s with different initial melt temperatures are always crystalline. The 1661 and 1441 bond-types or the bcc basic cluster (14 6 0 8) in the system play the critical role in the microstructure transitions. At the same time, it has been found that the effects of initial melt temperature on solidification microstructures are very remarkable, while they are not obvious in liquid and supercooled states, and the effects can be fully displayed only near the crystallization temperature Tc. The results also demonstrate that the effects of initial melt temperature on the crystallinity of solidification structures are different for different initial melt temperatures. Although the degree of influence is not linearly varying with the decrease of initial melt temperature, it still demonstrate that the influence degree can be controlled through the change of initial melt temperature. The cluster-type index method more clearly describes the cluster configurations in crystal system than the bond-type index method, so it is valuable to deeply investigate the evolution mechanisms of microstructures in liquid metal during solidification processes.