The structural characterization for Na+ Super Ionic Conductor (NASICON)-type glass ceramics of 20R(2)O-XZrO2-15P(2)O(5)-(65-X)SiO2 (R = Na, Li) was investigated by X-ray diffraction (XRD) and X-ray absorption fine structure to clarify the better conductive mechanism of Li-glass ceramics than that of Na-glass ceramics. The Na-glass ceramics were synthesized by heat treatment using glass material, and the Li-glass ceramics were obtained through ion exchange techniques from Na-glass ceramics. From the XRD analysis, the positive correlation between ionic conductivity and lattice parameter was identified for the Na-glass ceramics, on the other hand, there was no positive relation for Li-glass ceramics, indicating that the ion conductive mechanism cannot be explained by the lattice parameter of NASICON crystalline phase alone. The Debye-Waller factors related to the oxygen surrounding the Zr-ion for the Li-glass ceramics were higher than those for the Na-glass ceramics. The static disorder around Zr-ion may cause more weakly bonding between the Li ions and surrounding atoms, resulting in the improvement of ionic conductivity.