The method of solid-phase sintering was used to synthesize samples of lithium-conducting Li6.4Al0.2La3Zr2O12 solid electrolyte with a garnet structure. Higher technological efficiency of the method of synthesis was achieved owing to using low-melting nitrate crystalline hydrates of ZrO(NO3)2 · 2H2O and La(NO3)3 · 6H2O as initial substances: formation of the melt considerably intensified the interaction, caused a decrease in the duration and energy consumption of the method, and provided the presence of only a single phase of powdered solid electrolyte with a high Li-ion conductivity. The excess of the lithium-containing component (Li2CO3) in the mix was varied to compensate lithium losses in the course of the high-temperature treatment. Specific ionic conductivity of Li6.4Al0.2La3Zr2O12 was determined using the impedance spectroscopy technique and was 2 × 10–4 S/cm at the room temperature.