In this paper, the effect of Ge substitution for Nb on Li ion conductivity in the well-known Li5La3Nb2O12( LLN) parents with the cubic garnet framework prepared by using metal Ge is reported. The optimum condition is at 950 degrees C for 8 h with 10% excess lithium salt in air. The XPS spectra indicate that the metal Ge is completely transformed into Ge4+ after sintering for 8 h. The power XRD patterns reveal that wthe partial substitutions of Ge for Nb in Li5+ xLa3GexNb2- xO12 electrolytes are isostructural. The ionic conductivity of Li5+ xLa3GexNb2- xO12 ceramics increases as the molar fractions of Ge increasing for x <= 0.75. Among these compounds investigated, the Li5.75La3Ge0.75Nb1.25O12 exhibits the highest ionic conductivity of 1.2 x 10(-4)S cm(-1) at room temperature. The result can be regarded as relatively high values among the element substituted LLN garnets reported up to now. The new solid state electrolyte is very stable in potential range of -0.6-6.0 V vs. Li/Li+, indicating that Li5.75La3Ge0.75Nb1.25O12 is a promising solid state electrolyte for all-solid-state battery application. (C) 2017 Published by Elsevier Ltd.