Thin films of garnet-type Al-doped Li7La3Zr2O12 (LLZ) are prepared by the sol-gel process. Thin films are prepared on MgO substrates by a dip-coating process using a precursor sol from Zr-alkoxide and Li, La and Al nitrates. After the dip-coating, the dried films are calcined at 450 degrees C to get precursor films. When the precursor films are heat-treated at 900 degrees C in an alumina crucible, La2Zr2O7 is mainly obtained. With coexistence of Li2CO3 powders in the crucible during the heat-treatment at 900 degrees C, thin film of polycrystalline cubic LLZ is obtained. Addition of an ionic surfactant, lithium dodecylsulfate, improves the quality of the thin films, and the thin film heat-treated at 900 degrees C with coexistence of Li2CO3 powders in the crucible shows the ionic conductivity of 2.4 x 10(-6) S cm(-1) at 25 degrees C. (C) 2014 Elsevier B.V. All rights reserved.