Single crystals of Nd9.33(SiO4)6O2 and Sr2Nd8(SiO4)6O2 oxide ion conductors with the oxyapatite structure were grown without any macroscopic defect by the floating zone method. The oxide ion conductivity of the Sr2Nd8(SiO4)6O2 single crystal varied with the distance from its seed crystal along the growth direction, because of its changing deficiency in Sr content. Its stoichiometric portion had a lower electrical conductivity by about five orders of magnitude at 600 deg C than the value for Nd9.33(SiO4)6O2 single crystal. Structural refinement of neutron diffraction data for powdered Nd9.33(SiO4)6O2 single crystal showed that the previous structure analysis was misleading. Cation vacancies were present only at 4f site and its channel oxygen site was fully occupied in space group of P63/m. The oxygen had an anisotropic displacement along the channel in the refined oxyapatite structure.