Oxide solid electrolytes Li3/8Sr7/16-3x/2LaxZr1/4Ta3/4O3 (LSLZT, x = 0, 0.025, 0.05) with different A-site vacancy were synthesized using conventional solid-state reaction procedure at 1300 °C. Approximately single-phase perovskite-type was obtained which was analyzed by X-ray diffraction. Moreover, scanning electron microscope, AC impedance spectroscopy and potentiostatic polarization measurement methods were adopted to study the microstructure, Li+ conductivities and electronic conductivities of the samples, respectively. Among these samples, the optimal composition of Li3/8Sr7/16-3x/2LaxZr1/4Ta3/4O3 (x = 0.025) was selected with bulk conductivity of 1.26 × 10−3 S cm−1, total conductivity of 3.30 × 10−4 S cm−1, electronic conductivity of 6.60 × 10−9 S cm−1 at 30 °C and activation energy of 0.28 eV. Furthermore, the cyclic voltammogram analysis indicated the stability of this solid electrolyte at voltages higher than 1.3 V against metallic lithium. The solid electrolyte as a separator in LiFePO4/Li half-cell showed good cycle performance that comprises 98.7% of original values at 0.2 C charge-discharge rates after 50 cycles.