Lithium garnet-type oxides Li7-XLa3(Zr2-X, Nb-X)O-12 (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 degrees C in air. The lithium ion conductivity increased with increasing Nb content, and reached a maximum of similar to 0.8 mS cm(-1) at 25 degrees C. By contrast, the activation energy reached a minimum of similar to 30 kJ mol(-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. (C) 2010 Elsevier B.V. All rights reserved.