Na3Zr2Si2PO12 (NASICON) is a promising material as a solid electrolyte for all-solid-state sodium batteries. Nevertheless, one challenge for the application of NASICON in batteries is their high sintering temperature above 1200 degrees C, which can lead to volatilization of light elements and undesirable side reactions with electrode materials at such high temperatures. In this study, liquid-phase sintering of NASICON with a Na3BO3 (NBO) additive was performed for the first time to lower the NASICON sintering temperature. A dense NASICON-based ceramic was successfully obtained by sintering at 900 degrees C with 4.8wt% NBO. This liquid-phase sintered NASICON ceramic exhibited high total conductivity of 1x10(-3)Scm(-1) at room temperature and low conduction activation energy of 28kJmol(-1). Since the room-temperature conductivity is identical to that of conventional high-temperature-sintered NASICON, NBO was demonstrated as a good liquid-phase sintering additive for NASICON solid electrolyte. In the NASICON with 4.8 wt% NBO ceramic, most of the NASICON grains directly bonded with each other and some submicron sodium borates segregated in particulate form without full penetration to NASICON grain boundaries. This characteristic composite microstructure contributed to the high conductivity of the liquid-phase sintered NASICON.