Deuteron magnetic resonance (DMR) of single crystals of Cu(DCOO) 2 ·4D 2 O has been studied from 153°K to 291°K to reveal the changes in the structure and the motions of the crystalline water molecules due to the paraelectric-antiferroelectric phase transition ( T c =246.1±0.2°K). Above T c , all the DMR lines of water molecules are broadened out by fast motions of deuterons among seven sites. Below T c , 20 pairs of DMR lines are observed corresponding to four water molecules in stable position and two formate ions and their a c -plane enantiomorphs. Following conclusions are derived from measurements of temperature and angular dependences of the quadrupole splittings for these DMR lines. The phase transition is an order-disordering type of water molecules. In the antiferroelectric phase, the unit cell doubling occurs along the c -axis direction and the space group is P 2 1 / n . The structure involves a three dimensional ordering of water molecules. The structure of hydrogen-bond network is determined. Each two-dimensional layer of water molecules has a ferroelectric structure and the two successive layers in a unit cell arrange anti-parallel. Torsional motion of formate ion, DCOO - , is activated around the O…O direction.