Ceramic sheet of Nathorn ion-conducting solid electrolyte, Na3Zr2Si2PO12 (NASICON), has been prepared by tape-casting method. Addition of 60Na(2)O-10Nb(2)O(5)-30P(2)O(5) glass as a sintering aid is crucial for fabrication of fully-densified impermeable sheet. Otherwise the gas tightness is not achieved even sintered at 1200 degrees C. The best preparation condition is 5 or 10 wt% grass-addition with respect to NASICON and sintering at 1000-1100 degrees C, which is lower by 100-200 degrees C than the typical densification temperature of NASICON without the additive. A sample with a thickness of 35 mm and total conductivity of 4.4 x 10(-4) S cm(-1) is obtained by 5 wt% glass addition and sintering at 1100 degrees C for 4 h. (C) 2018 Elsevier Ltd. All rights reserved.