Lanthanum silicates are promising candidates for SOFC electrolytes for their high oxide ionic conductivity at moderate temperatures. One of the main limitations of lanthanum silicate materials resides in the difficulty in preparing dense ceramics. In this work, a hydrothermal method has been developed for the synthesis of Na-doped lanthanum silicates, with a chemical formula of NaLa9(SiO4)6O2. Experimental results show that the synthesis is strongly dependent on the experimental parameters, including temperature, reaction time and pH value. Under the optimal synthesizing conditions, high-crystalline NaLa9(SiO4)6O2 nanoparticles with 74.7 nm in diameter and narrow size distribution can be obtained. The obtained fine powders show good sintering activity and can be pressureless sintered to 94.8 % of theoretical density at 1500 °C. Furthermore, the sintered bodies of NaLa9(SiO4)6O2 show ionic conductivity of 1.01 mS/cm at 700 °C, with an activation energy of 0.84 eV.