Nucleation of NaCl was reproduced by using molecular dynamics simulation. We carried out MD simulations in the 13 824 ion system under the free boundary condition. Critical nucleus size, nucleation time lag, and nucleation rate were directly estimated from simulation results without using nucleation theory. We also carried out MD simulations in the 13 824 and 125 000 ion systems under the periodic boundary condition to compare the results with those of the free boundary condition. A single crystal and a polycrystal were formed in our simulation. We investigated the difference of the nucleation process of a single crystal and a polycrystal. All simulations were calculated by using the special purpose computer, MDGRAPE-2, for molecular dynamics simulation. (C) 2003 American Institute of Physics.