In order to observe the influences on microscopic behavior of water molecules caused by water treatment processes, the changes of dynamic structure of water in an ultra pure water (UPW) producing system have been studied by dielectric and O-17-NMR relaxations. The UPW system used in this study consists of typical unit processes employed in current commercial systems for semiconductor manufacturing plants, and produces UPW of the highest quality with specific resistivity of 18.2 M Omega . cm. Throughout the whole system from city water till UPW dielectric relaxation time, tau(d), is kept constant at about 8.7 ps, while beta, a parameter describing the distribution of tau(d) increases with each treatment step, reaching a constant value of about 1.00 after the removal of major impurities by a reverse osmosis membrane. The spin-lattice relaxation time, T-1, obtained from O-17-NMR is unchanged at about 7.3 ms through all processes in the system. Spin-spin relaxation time, T-2, shows drastic changes with each treatment step. The changes of T-2 are well explained by proton-exchange reactions in water and should not be related directly with rotational motions of water molecules. These results indicate that the averaged rotational motion of water molecules is unchanged from city water to UPW. The distribution around the average, however, becomes narrower during purification, and the rotational motions of water molecules are highly homogenized in UPW.