The stability and structure of Na(H2O)(n) (n = 1-6) as well as their ions have been investigated with the nb initio MO method. Both the surface structure where Na is situated on the surface of the water cluster and the interior structure where Na is surrounded by water molecules are minima on the potential surface. They are very close in energy, though the surface structure becomes dominant as n increases. The surface structure is stabilized mainly by the solvent-solvent interaction, while the interior structure is stabilized by the Na-solvent interaction. In the surface structure the SOMO electron is localized in the vicinity of Na opposite to the hydrating water molecules, whereas in the interior structure the SOMO density spreads into the region between water molecules. This difference is responsible to the difference in the n dependency of the ionization potential.