Lithium ionic conductors which exhibit high ionic conductivity and are stable to lithiummetal are demanded as solid electrolytes of all solid-state lithium batteries. Garnet-type lithium ionic conductor, Li6.25+xGa0.25La3-xSrxZr2O12 (LLZ-GaSr) has been synthesized at 1000 degrees C by a solid-state reaction. Its phase relation, sinterability, structure and ionic conductivities have been investigated. LLZ-GaSr forms cubic phase in x = 0-0.2 as main phase, while it forms tetragonal phase with impurities in x = 0.3-0.5. XRD studies indicate the solubility limit in cubic phase of LLZ-GaSr is around x = 0.1. Sinterability is improved by Sr substitution. The total ionic conductivity of LLZ-GaSr with x = 0.1 at 25 degrees C exhibits 1.3 x 10(-3) S cm(-1). The symmetrical cell with Li electrodes demonstrates that LLZ-GaSr is stable in Li metal at room temperature for 1.5 month. The Sr-substitution is effective in improvements of sinterability and ionic conductivity of Li6.25Ga0.25La3Zr2O12 (LLZ-Ga). (C) The Author(s) 2018. Published by ECS.