Some clay minerals are used and expected as barrier materials for engineering and in nature. Hydrotalcite, Mg₆Al₂[(OH)₁₆|CO₃]4H₂O and a kind of LDH (layered double hydroxides), is one of the most effective candidates for the anion adsorbents and the barrier. In this study, the behavior of hydrotalcite was investigated by means of the molecular dynamics method. Cl⁻ and I-hydrotalcite − water systems were simulated for various mineral/water ratios. The structure and dynamic properties are predicted. Water at the surface of hydrotalcite shows the electric double layer composed of Stern layer of one H₂O molecular layer thickness and large self-diffusion coefficient of H₂O and diffusion layer of 2.5 nm thickness at the interface.