Abstract Thin films of La 1.61GeO 5− δ , a new oxide ionic conductor, were fabricated on dense polycrystalline Al 2O 3 substrates by a pulsed laser deposition (PLD) method and the effect of the film thickness on the oxide ionic conductivity was investigated on the nanoscale. The deposition parameters were optimized to obtain La 1.61GeO 5− δ thin films with stoichiometric composition. Annealing was found necessary to get crystalline La 1.61GeO 5− δ thin films. It was also found that the annealed La 1.61GeO 5− δ film exhibited extraordinarily high oxide ionic conductivity. Due to the nano-size effects, the oxide ion conductivity of La 1.61GeO 5− δ thin films increased with the decreasing thickness as compared to that in bulk La 1.61GeO 5− δ . In particular, the improvement in conductivity of the film at low temperature was significant .The electrical conductivity of the La 1.61GeO 5− δ film with a thickness of 373 nm is as high as 0.05 S cm − 1 (log( σ/S cm − 1 ) = − 1.3) at 573 K.