Thermal poling has been applied to 2Ag2O ·3Na 2O ·25ZnO ·70TeO2 (in mol%) glass sandwiched in between two commercially-available cover glasses, and precipitation of Ag nanoparticles has been induced selectively in the vicinity of anode-side glass surface. Migration of Na+ during the thermal poling has been investigated by means of energy dispersive x-ray spectroscopy (EDX) to clarify the mechanism of Ag precipitation. The elemental analysis on the surfaces of cover glasses and tellurite glass samples using EDX indicates that the drift velocity of Na+ in the tellurite glass is lower compared to that in cover glass, suggesting that Na+ ions accumulate near the anode-side surface of tellurite glass to facilitate the reduction of Ag+ to Ag. Also, the radius and volume fraction of nanoparticles are evaluated by theoretical fit using Mie scattering theory and Drude free electron theory, and the variation of size and amount of Ag nanoparticles with poling conditions such as poling temperature, time, and voltage is discussed.