Crystal structures and magnetic properties of the ternary oxides Ln 3NbO 7 ( Ln=La, Pr, Nd, Sm–Lu) are reported. Their powder X-ray diffraction measurements and Rietveld analyzes show that they have the fluorite-related structures with space group Pnma ( Ln=La, Pr, Nd), C222 1 ( Ln=Sm–Tb), or Fm-3 m ( Ln=Dy–Lu). Magnetic susceptibility measurements were carried out from 1.8 to 400 K. The Ln 3NbO 7 compounds for Ln=Pr, Gd, Dy–Yb show Curie–Weiss paramagnetic behavior, and Sm 3NbO 7 and Eu 3NbO 7 show van Vleck paramagnetism. On the other hand, two magnetic anomalies were observed for both Nd 3NbO 7 (0.6 and 2.7 K) and Tb 3NbO 7 (2.0 and 3.2 K). From the results of specific heat measurements, it was found that these anomalies are due to the antiferromagnetic ordering of Ln ions in two different crystallographic sites (the 8-coordinated and 7-coordinated sites).