Sm2O3 optical thin films were prepared by hydrothermal method using samarium dichloride as source material. The influence of preparation temperature on the crystallinity, morphology and optical properties of Sm2O3 thin film were investigated. The as-fabricated thin films were characterized by X-ray diffraction, atomic force microscopy and spectrophotometer. Results show that the prepared thin film is Sm2O3 crystallites thin film with compact and uniform surface microstructures. Transmittance curve displays that the Sm2O3 crystallites thin films are transparent to visible and infrared light; and possess excellent absorbency of UV light. With the increase of deposition temperature, the crystallinity and optical properties of the prepared Sm2O3 thin film are improved. With the increase of hydrothermal temperature from 170 °C to 185 °C, the bandgap of the thin film increase from 2.88 eV to 2.95 eV.