Li1.5Al0.5Ge1.5(PO4)(3) (LAGP) is a promising solid electrolyte for all-solid-state Li ion batteries. In this work, it was synthesized using solid-state reaction with an excess amount of Li to improve grain boundary ionic conductivity. The additional Li improved grain boundary conductivity even though the relative density decreased. This improvement may originate from a beneficial characteristic of grain boundary induced by the segregation of some of Li to the grain boundary. This segregation was indirectly observed by the change of morphology of particles in samples with excess Li. The segregation of Li may result in a facile Li transport in grain boundaries, as indicated by low activation energy and a high pre-exponential factor of the grain boundary conductivity. Through improving grain boundary conductivity with excess Li, a high total ionic conductivity of 1.9 x 10(-4) S.cm(-1) is achieved at room temperature even with a low relative density of 78%. This porous and high ionic conducting solid electrolyte can be useful in configuring the electrode composite of all-solid-state cells. (C) 2014 Elsevier B.V. All rights reserved.