Low-temperature deposition of Y₂O₃ at 80^°C is studied using an yttrium precursor of tris(butylcyclopentadienyl)yttrium (Y(BuCp)₃) and plasma exited humidified argon oxidizer. The deposition is demonstrated using an atomic-layer-deposition sequence; the Y(BuCp)₃ and the oxidizing gases are time separately introduced to the reaction chamber and these injections are repeated. To determine the gas introduction conditions, surface reactions of Y(BuCp)₃ adsorption and its oxidization are observed by an in-situ IR absorption spectroscopy. The deposited film is confirmed as fully oxidized Y₂O₃ by X-ray photoelectron spectroscopy. The present deposition is applicable for the deposition of Y₂O₃ film on flexible polyethylene terephthalate films.