Crystal structures of the Ln 2 Zr 2 O 7 (Ln = Nd and La) were refined by the Rietveld analysis of powder neutron diffraction (ND) data at room temperature. La 2 Zr 2 O 7 had a pyrochlore (P)-type structure of almost completely ordered for oxygen vacancy. On the other hand, Nd 2 Zr 2 O 7 had the P-type structure for the oxygen vacancy distributed between the O2 (8b) and O3 (48f ) sites. The Rietveld refinement results of powder X-ray diffraction (XRD) data of Ln 2 Zr 2 O 7 (Ln = Eu, Nd and La) indicated that the Ln 3+ and Zr 4+ ions exist in ordered states in 16c and 16d sites, respectively. The refined lattice parameters (a), occupancies (g) for oxygen sites (O1, O2 and O3) and x-positional parameters for O3 (48f ) site of La 2 Zr 2 O 7 and Nd 2 Zr 2 O 7 by using powder XRD data were in almost agreement with those by using powder ND data. The refined ND and XRD data of Ln 2 Zr 2 O 7 (Ln = Eu, Nd and La) system revealed that the O3 (48f ) site occupancy (g 48f ) decreases with increasing O2 (8b) site occupancy, while O1 (8a) site occupancy keeping 1.0. The change in oxide-ion conductivity (·) in Ln 2 Zr 2 O 7 (Ln = Eu, Nd and La) system can be related to a change in the product of site occupancy (g 48f ) and the oxygen vacancy rate (1-g 48f ) in the O3 (48f ) site, g 48f (1-g 48f ).