Recently, as the spread of electric vehicles increases, the need for energy storage devices having high energy density and safety has been highlighted. However, it is currently limited due to insufficient safety performance due to the use of flammable liquid organic electrolyte in the lithium ion battery. Therefore, many researches for the replacement of liquid electrolyte into the solid state electrolyte have been tried in various ways. Among them, Li1+xAlxTi2-x(PO4)3 (LATP) electrolyte is one of the promising solid electrolyte candidates due to its high ionic conductivity and excellent chemical stability. However, practical application is difficult due to problems such as poor contact with electrodes, severe side reactions, and still low ionic conductivity. In this study, we have successfully synthesized the LATP electrolyte with high ionic conductivity by incorporating GeO2. During sintering process, the liquid phase of GeO2 helps to enhance the density, and remained phases between the grain boundaries reduced the grain boundary resistance effectively. In addition, the high ionic conductivity gives high cell performance, where the overpotential of symmetry cell (Li/electrolyte/Li) reduced significantly after GeO2 incorporation. This study shows that GeO2 doping is effective for improving LATP solid state electrolyte.