NASICON-structured Li1.5Al0.5Ge1.5(PO4)3 glass-ceramic is successfully fabricated through melt-quench with post-crystallization. The influence of crystallization temperature on structure and ionic conductivity is studied using X-ray diffraction, nuclear magnetic resonance and impedance analysis, revealing that the Li ion mobility is closely related with crystallization temperature that in turn affects the bulk conductivity. Scanning electron microscopy reveals a progress of transformation from an amorphous phase to a crystalline, and grain growth during the annealing. It is shown that crystallization temperature plays an important role in controlling the ionic conductivity. The highest conductivity of 2.25 × 10−3 S cm−1 with an activation energy of 0.29 eV at room temperature has been obtained for the glass-ceramic specimen crystallized at 825 °C for 8 h.