Oxide ion conductivity inLa(10)Si(6)O(27) and La10Ce6O27-based apatite oxides was investigated in this study. In spite of the low symmetry of the crystal lattice, both oxides exhibited high oxide ion conductivity over a wide range of oxygen partial pressures. Oxide ion conductivity of La10Si6O27 was increased by doping Sr fur La sites. On the other hand, it was found that La10Ge6O27 also exhibited high oxide ion conductivity which is comparable with that of La0.9Sr0.1Ga0.8Mg0.2O3 perovskite oxide. Arrhenius plots of the oxide ion conductivity in La10Ge6O27 show a knee around 1000 K. However, the knee in the Arrhenius plot of conductivity disappeared by doping Sr for La sites. Thus, the conductivity at low temperature was greatly enhanced by doping Sr for La sites in La10Ge6O26. This may be due to stabilization of the high temperature crystal phase. The electrical conductivities of Sr-doped La10Si6O27 and La10Ge6O27 were almost independent of the oxygen partial pressure from P-O2 = 1 to 10(-21) atm. This suggests that these materials are ionic over a wide P-O2 range. This study revealed that apatite oxide of La10M6O27 (M - Si, Ge) is a new class of the fast oxide ion conductors.