Molten sodium (Na) anode high temperature batteries, such as Na-NiCl2 and Na-S, draw attentions to be used in stationary electricity storage applications. Recent efforts are exerted to lower their operating temperatures down to below 200 °C in order to adopt ultra-low cost cell production, establish easier maintenance, pursue enhanced safety, and more. One of main challenges in lowering the operation temperature is radical decrease in ionic conductivity of their solid electrolytes. Na3Zr2Si2PO12 (Nasicon) is considered as a solid electrolyte for the lower temperature operation. Here we report Na ionic conductivity of Nasicon at 150 °C increases by adding Ge element. The ionic conductivity of Ge-added sample (Na3[Zr2-δGeδ]Si2PO12, δ = 0.1, 0.2) is measured as high as 1.4 × 10−2 S cm-1 at 150 °C which is about two times higher than those of the bare Nasicon. The phase transition temperature of the Ge-added samples is lowered, thereby the volume fraction of the rhombohedral phase, which is stable at higher temperatures and exhibits higher Na ion conductivities, increases. This finding provides a useful guideline to further increase the ionic conductivity of Nasicon solid electrolytes, which can advance materialization of lower temperature operating Na batteries.