The ionic conductivity of the Li2O–B2O3–SiO2 glass system was studied for potential application as a thin film electrolyte and compared with that of the Li2O–B2O3–P2O5 glass system. Among oxide glasses, these glass electrolytes are believed to be the most suitable for a thin film electrolyte due to the stable amorphous glass, its inertness to moisture and the ease of fabrication. The effect of spinodal decomposition on the ionic conductivity in the Li2O–B2O3–SiO2 glass system was also discussed. The ionic conductivity of phase-separated glass is dependent on the microstructure of the highly conductive phase. The structural analysis of the glass electrolyte was performed with various chemical compositions and the correlation to the ionic conductivity is discussed. The maximum conductivity of the Li2O–B2O3–SiO2 glass system is 2.4×10−8 Scm−1, which is lower than that of the Li2O–B2O3–P2O5 glass system by almost one order of magnitude. However, despite low conductivity, the Li2O–B2O3–SiO2 glass system is still believed to be a strong candidate for a thin film electrolyte since its glass formability and chemical stability are excellent compared to other glass electrolyte systems.