Li7P3S11 (LPS) solid electrolytes have great advantages, such as high lithium-ion conductivity, and can easily fabricate to make bulk-type all-solid-state lithium batteries. However, LPS has a disadvantage because of large interfacial resistance with lithium, which give a limitation for all-solid-state battery. In this study, (100-x)Li7P3S11-x Li3SI (x = 0. 2 and 5) solid electrolyte composites are prepared by mechanical ball milling process. Various physiochemical analysis was carried out to confirm the proper mixing of Li7P3S1 and Li3SI solid electrolytes. The addition of Li3SI solid electrolyte, effectively influence the ionicconductivity and electrochemical properties of the Li7P3S1 solid electrolyte. The prepared Li7P3S11/Li3SI solid electrolyte composites are stable against the lithium metal anode even after 100 h chargedischarging process and has stable potential window up to 10 V. In particular, 98Li7P3S11-2Li3SI solid electrolyte based ASSLBs shows the highest capacity, of 110 mA/g at the current density of 7.5 mA g-1 (0.05 C). The charge - discharge cycle stability test also proved the Li3SI mixing to Li7P3S11 helped to maintain the capacity retention value of ~100 % after 10 cycles. Therefore, the Li3SI into the Li7P3S11 electrolyte helps improve the electrochemical performance.