Molecular dynamics simulations are performed on nano clusters with 500 nickle atoms (Ni500) to determine the structure and dynamics in the isothermal crystallization using embed atom model potentials. Through the study of nano Ni cluster dynamic behavior, we find that multi-step crystallization process of Ni500 is at temperatures far below the melting temperature (Tm), whereas one-step crystallization process is at the temperatures just below Tm. Multi metastable states are observed before Ni500 becomes fully crystallized in multi-step crystallization process. We find that atom rearrangements take not only string-like cooperative motion manner but also other collective motion manner.