Solid electrolytes are strong candidates for next-generation Li-ion bat-teries for Electrical Vehicle (EV) applications. However, their usage is not widely spread because of their relatively poor ionic conductivity and high electrode/electrolyte interfacial resistance compared to their com-mercial counterparts, organic based liquid electrolytes. Sodium Superi-onic Conductor (NASICON) type solid electrolytes could be an option to overcome these problems. Li1.5Al0.5Ge1.5(PO4)3 (LAGP) stands forward among other NASICON type solid electrolytes with their easy synthesis and processing. Yet, their stability against various mediums remain un-known and needs to be enlightened. In this study, the stability of LAGP against water, air at 85°C and 1M LiOH solution was discussed. LAGP re-sults after sintering at 900°C for 3 hours showed impurity-free, highly dense structure with a restricted grain growth. Water and air aged sam-ples showed a dramatical reduction on grain boundary contribution of to-tal ionic conductivity whereas the sample aged at 1M LiOH solution ex-hibited both reduction in ionic conductivity and increase on grain boundary conductivity. The highest reduction on total ionic conductivity was observed on the sample aged in water. On the contrary, the sample aged in 1M LiOH solution resulted in a net increase on total ionic con-ductivity. The highest total conductivity of 4.3 x 10-4 S/cm was obtained from the sample aged in 1M LiOH. On the other hand, the sample aged the lowest conductivity – 1.8 x 10-4 S/cm – showed the lowest total con-ductivity of all samples.