Glass-ceramics of the vanadium- or molybdenum-containing Na5RSi4O12-type (R = rare earth; Y) Na+-superionic conductors were prepared by crystallization of glasses with the compositions Na3+3x-yY1-xVySi3-yO9 (NYVS) or Na3+3x-2yY1-xMoySi3-yO9 (NYMS), and the effects of V or Mo elements on the separation of the phase and the microstructural effects on the conduction properties of glass-ceramics were discussed. The conductivities of the glass-ceramic NYVS and NYMS with the Na3.9Y0.6V0.3Si2.7O9 and Na3.7Y0.7Mo0.1Si2.9O9 compositions were 0.87 x 10(-2) and 3.58 x 10(-2) S/cm at 300 degrees C, respectively, and the Na+ ionic transport numbers of these glass-ceramics determined by Wagner polarization method were nearly 0.9 and 1 at 300 degrees C, respectively. (C) 2008 Elsevier B.V. All tights reserved.