We studied the effects of crystallization conditions on the conduction properties of the Na₅YSi₄O₁₂-type sodium ion conducting glass-ceramics. Crystallization process was controlled by varying annealing parameters of nucleation time (0–200 h) and crystal growth temperature (900–1100 °C). The present results showed that a shorter nucleation time with annealing at 900 °C for crystal growth gave rise to higher conductivities of 5 × 10⁻² S cm⁻¹ at representative temperatures of 300 °C and 3 × 10⁻³ S cm⁻¹ at 150 °C, respectively. Those results were attributed to the microstructures of glass-ceramics and the conductivities of a whole glass-ceramic were considered to be dominated by those of grains at 300 °C and, on the other hand, strongly influenced by those of grain boundaries at 150 °C. It was found that the dependence of a whole conductivity on the apparent number of grain boundaries calculated as the ratio of a whole thickness to an average grain diameter was stronger in the measurements at 150 °C than at 300 °C. Considering the result, we propose the use of largely grained glass-ceramics for both higher and lower temperature uses.