We have successfully synthesized the highly c-axis-oriented polycrystals of apatite-type lanthanum silicate by the reactive diffusion technique. When the La 2 SiO 5 /La 2 Si 2 O 7 diffusion couples were isothermally heated at 1773−1873 K for 5−100 h, the apatite polycrystals were readily produced in the form of a layer at the interfacial boundaries. The annealed couples were characterized using optical microscopy, micro-Raman spectroscopy, X-ray diffractometry, and electron probe microanalysis. The product layers were composed of the highly c-axis-oriented prismatic crystallites, with their elongation directions being almost parallel to the diffusion direction. The formation of the apatite layer was controlled by volume diffusion, the overall reaction of which is described by (10 + 6x)La 2 SiO 5 + (4−3x)La 2 Si 2 O 7 → 3La 9.33+2x (SiO 4 ) 6 O 2+3x (0.01 ≤ x ≤ 0.13). The apatite layer formed at 1873 K was characterized by the steady decrease of the x-value along the diffusion direction from 0.13 at the La 2 SiO 5 /apatite interface to 0.01 at the apatite/La 2 Si 2 O 7 interface. We have also prepared sandwich-type La 2 Si 2 O 7 /La 2 SiO 5 /La 2 Si 2 O 7 diffusion couples and heated them at 1873 K for 100 h. The annealed couple was mechanically processed, and the thin-plate electrolyte consisting of the highly c-axis-oriented polycrystal was obtained. The oxide-ion conductivity was determined from the impedance spectroscopy data at 573−973 K, which steadily increased from 2.4 × 10 −3 S/cm to 2.39 × 10 −2 S/cm with increasing temperature. The empirical activation energy of conduction was 0.35 eV, which compares well with the calculated migration energy of 0.32 eV in a previous study.