Ultrafine SnO2 thin films have been prepared by the hydrothermal method. Pretreatment at a lower temperature (60-100 degrees C) was found to be necessary for SnO2 nuclei on the Si(100) substrate. The X-ray diffraction pattern showed that the films with an average particle size of 3.5 nm were polycrystalline. The scanning electron micrograph of the cross-section showed a homogeneous layer of 0.2 mu m film formed on the substrate. X-ray photoelectron spectroscopy and gas-sensitivity properties of the film were also investigated.