Tin dioxide thin films were prepared by dual ion beam sputtering on various substrates. The Sn and the SnO2 targets were sputtered with 1000 eV. 1.5-2.0 mA/cm(2) argon ion beam, and the water-cooled substrates were simultaneously bombarded with oxygen ion beam of 0-200 eV and 200 mu A/cm(2). All of oxygen-assisted thin films showed high transparency and bulk-like refractive indices, while the sputtered film without bombardment had relatively poor transmittance, As the bombarding energy increased, the absorption edge shifted to lower wavelength and the optical band gap energy increased to a maximum value of 4.21 eV, Four-point-probe measurement nt showed that the resistivity of as-deposited films depended strongly on target material and bombardment energy. After heat treatment, the optical band gap and the resistivity showed similar behavior and had their maxima after annealing at 400 degrees C.