Garnet-type Ta-doped Li7La3Zr2O12 (LLZO) ceramic solid electrolytes with Ga2O3 additive were synthesized via a conventional solid-state reaction process. When the amounts of Ga2O3 additive were below 2 mol %, the sintered sample has a dense structure composed of grains with the averaged size of 5 to 10 μm, while 3 mol % or more Ga2O3 addition causes the significant increase in grain size above several 10 to 100 μm, due to the sintering with large amount of liquid Li-Ga-O phase at high temperature. The highest total (bulk + grain-boundary) ionic conductivity of 1.1 mS cm1 at room temperature was obtained in the sample with 5 mol % Ga2O3 addition. However, in galvanostatic testing of the symmetric cell with Li metal electrodes, this sample was shorted by Li dendrite growth into solid electrolyte at current density below 0.2 mA cm2. The tolerance for Li dendrite growth is maximized in sample sintered with 2 mol % Ga2O3 addition, which was shorted at 0.8 mA cm2 in the symmetric cell. Since the interfacial resistance between Li metal and solid electrolyte was nearly identical among the all samples, the difference in tolerance for Li dendrite growth is mainly attributed to the difference in microstructure of sintered samples depending on the amounts of Ga2O3.