In recent years there has been a resurgence in the use of Li metal as an anode, because such use can to lead to rechargeable batteries with higher energy than currently exist. Using a Li anode with a liquid electrolyte can lead to dendrite shorting and flammability issues. To overcome these problems Li-ion conducting solid electrolytes are attracting considerable attention. The major requirements for the solid Li-ion conducting electrolyte are: high Li-ion conductivity, low electronic conductivity, high relative density, chemical/electrochemical stability with the Li anode and cathodes and adequate mechanical properties. The ionic, electrochemical, and mechanical properties of the solid-state electrolyte are in general a function of the microstructure. For a single-phase material the major microstructural variables are porosity and grain size. For a solid-state electrolyte a high relative density is required thus, the major microstructural variable is the grain size. It is the purpose of this presentation to discuss the effect of grain size (5 to 600 mm) on the room temperature total ionic conductivity, hardness, fracture toughness and critical current density (current above where Li plating occurs) for hot-pressed high relative density (>97%) cubic Al-doped Li₇La₃Zr₂O₁₂ with nominal composition of Li_6.25Al_0.25La₃Zr₂O₁₂.