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Crystal structures of monoclinic RbD 2 PO 4 were studied at 297 K in the ferrielectric phase (phase III) and at 350 K in the paraelectric phase (phase II) by means of the single crystal neutron diffraction. The oxygen and deuterium atoms associated with the hydrogen bonds along the b axis are ordered in phase III and are disordered in phase II. Other atoms are ordered in both phases II and III. There are four kinds of PO 4 tetrahedra in phase III. The ferrielectricity of phase III were confirmed in the present neutron structural analysis. The III-II phase transition is caused by an order-disorder of PO 4 tetrahedra accompanied by the motion of deuterium atoms within the hydrogen-bond chain. Fluctuations of the oxygen and deuterium atoms related hydrogen-bond chains along the b axis give rise to a quasi-onedimensionality.