NASICON-like Na 3 Zr 2 (SiO 4 ) 2 PO 4 (NZSP) ceramic solid electrolyte with high ionic conductivity, safety, and durability becomes the main focus and attention as an alternative for traditional liquid electrolytes. NZSP containing NH 4 H 2 PO 4 and Na 3 PO 4 ⋅12H 2 O as the phosphate source has been extensively studied as a solid electrolyte, but a deep understanding of the relationship between crystal growth and ionic conductivity is still lacking. Herein, we synthesized NZSP via a solid-state reaction using NaH 2 PO 4 as the phosphate source. The impact of different sintering holding times on the crystal phase, microstructure, ionic conductivity, and relaxation time of NZSP solid electrolytes was investigated. Microstructure studies revealed that the faceted NZSP sintered at 1100 °C for 24 h has the lowest formation of ZrO 2 and highest densification with the least pores. In addition, the sample achieved the highest room temperature ionic conductivity (4.11 ⨯ 10 −4 S cm −1 ) and the shortest relaxation time (0.4 μs), which are also crucial factors for the development of rechargeable all-solid-state batteries (ASSBs).