Water-stable and high lithium-ion conducting solid electrolytes have potential application as the electrolyte of solid state batteries and the separator of high energy density aqueous lithium batteries. The lithium-ion conductivity of the water-stable NASICON-type solid electrolyte of Li1.4Al0.4Ge0.2Ti1.4(PO4)(3) (LAGTP) was enhanced by addition of TiO2 and Al2O3. The highest conductivity of 1.53 x 10(-3) S cm(-1) at 25 degrees C was observed for LAGTP-3 wt% Al2O3, which is approximately 1.5 times higher than that of LAGTP. The conductivity enhance ment could be explained by a reduction of the grain boundary resistance. The three-point bending strength was also increased by addition of Al2O3, TiO2, and ZrO2. The LAGTP-10 wt% TiO2 composite showed a bending strength of 209 N mm(-2), which was 3 times higher than that of LAGTP, and the conductivity was comparable to that of LAGTP.