Abstract: Sodium montmorillonite is a layered clay mineral composed of hydrous aluminum silicate. The hydration and microscopic physical/mechanical properties of the mineral were investigated using a molecular dynamics simulation in the current study. After the model containing various water molecules of the sodium montmorillonite were developed, the crystal cell parameters and the various types of the energies before and after energy optimization were then analyzed. The changes in the volume and density with various water molecules and temperatures were also investigated. The hydration of the mineral was thereafter examined using the radial distribution function. The physical and mechanical properties of the mineral with various water molecules were further more explored. It shows that the shape of the unit cell of the sodium montmorillonite became much irregular after the energy optimization; the trosion energy increases, whereas other types of energies and the total energy decreased at different levels after the energy optimization; with the increase in water molecules, the volume and density of the mineral would increase and decrease respectively; as the surrounding temperature rose, the volume and density of the mineral would increase and decrease respectively; the hydration and mechanical properties of the mineral were much related to the number of water molecules.