Two types of apatite-type lanthanum silicates, La 9.93 (Si 5.11 Mg 0.89 )O 26 (space group P6 3 /m) and (La 8.48 Mg 1.28 )Si 6 O 26 (space group P6 3 ), were successfully synthesized. They were, together with La 9.33 Si 6 O 26 (space group P6 3 /m), examined by optical microscopy, impedance spectroscopy and X-ray powder diffraction. The crystal structures of the former two compounds were refined by single-crystal X-ray diffraction. The oxide-ion conductivity at 773K was the highest for La 9.93 (Si 5.11 Mg 0.89 )O 26 , followed by La 9.33 Si 6 O 26 and (La 8.48 Mg 1.28 )Si 6 O 26 in this order. The systematic change in conductivity was well accounted for by the atom arrangements of hexagonal channels, consisting of alternately stacking equilateral triangles of La and O along the c-axis. With La 9.93 (Si 5.11 Mg 0.89 )O 26 , the La triangle appreciably expanded as compared with that of La 9.33 Si 6 O 26 , while the size of O triangle was kept almost constant. When the La sites were partially occupied by Mg atoms, the space group changed from P6 3 /m to P6 3 , leading to the transformation of equivalent O triangles into the larger and smaller ones. On the assumption that conduction takes place inside the hexagonal channels through the migration of channel oxide-ions, they necessarily pass through the channel triangles. The smaller O triangle would act as a bottleneck hindering the migration of channel oxide ions. The expansion of La triangle would effectively enhance the conductivity.