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 °C to get precursor films. When the precursor films are heat-treated at 900 °C in an alumina crucible, La2Zr2O7 is mainly obtained. With coexistence of Li2CO3 powders in the crucible during the heat-treatment at 900 °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 °C with coexistence of Li2CO3 powders in the crucible shows the ionic conductivity of 2.4 × 10−6 S cm−1 at 25 °C.