A new thin films formation technique using gas-cluster ion beams is demonstrated. Oxygen cluster ion beams are utilized to grow oxide films. We have found that high quality SiO2 films of up to 11 nm thick can be formed on Si substrate surfaces at room temperature by direct oxidation using O2 and CO2 cluster ion beams at acceleration voltages of less than 10 keV. O2 cluster ions were also used to irradiate Si substrates covered with SiO2 films, during the evaporation of Pb atoms. Increasing the acceleration voltages, the O2 cluster ion bombardment enhanced the oxidation and surface smoothing. At the acceleration voltages above 5 kV, the polycrystalline lead oxide films were oriented with their (111) direction parallel to the surface. A significant smoothing effect was observed, even if the acceleration voltage was as low as 1 kV. An average surface roughness of 0.9 nm was obtained at 7 kV. These results indicate that O2 cluster ions strongly enhanced the oxidation and the smoothness without causing irradia...