In this paper, the effect of Na substitution for La on Li ion conductivity in the well-known Li5La3Nb2O12 parents with the cubic garnet framework prepared by sol-gel process is reported. The optimum condition is at 750 degrees C for 6 h in air with 10% excess lithium salt. The power XRD patterns reveal that the partial substitutions of Na for La in Li5 + 2(x)La(3-x)Na(x)Nb(2)O(12) electrolytes are isostructural. The cubic lattice parameters decrease as the molar fractions of Na increasing. Among the Li5 + 2(x)La(3.0)Na(x)Nb(2)O(12) compounds investigated, the Na-substituted Li6.4La2.3Na0.7Nb2O12 exhibits the highest ionic conductivity of 1.6 x 10(-4) S cm(-1) at room temperature with an activation energy of 0.41 eV. The new solid state electrolyte is very stable in potential range of -0.8 to 5.0 V vs. Li/Li+, and is also chemically stable against LiMn2O4 and LiCoO2 compounds up to 900 degrees C, indicating that Li6.4La2.3Na0.7Nb2O12 is a promising solid state electrolyte for all-solid-state battery application.