Lithium garnet-type oxides Li7−X La3(Zr2−X , Nb X )O12 (X =0–2) were synthesized by a solid-state reaction, and their lithium ion conductivity was measured using an AC impedance method at temperatures ranging from 25 to 150°C in air. The lithium ion conductivity increased with increasing Nb content, and reached a maximum of ∼0.8mScm−1 at 25°C. By contrast, the activation energy reached a minimum of ∼30kJmol−1 at the same point with X =0.25. The potential window was examined by cyclic voltammetry (CV), which showed lithium deposition and dissolution peaks around 0V vs. Li+/Li, but showed no evidence of other reactions up to 9V vs. Li+/Li.