High-quality Ta2O5/SiO2 were deposited with oxygen gas cluster ion assisted deposition at low-temperature for advanced optical filters. With gas cluster ion assisted deposition, high refractive index and very smooth surface of Ta2O5 films were realized. The optimum cluster ion energy and cluster ion current density for Ta2O5 films were found to be 7keV and 0.5μA/cm2, respectively. The structure of film was very uniform and no porous or columnar structures were observed. The surface or interfaces of Ta2O5/SiO2 films were also very flat by surface smoothing effect of cluster ion beams. Very smooth surface can be realized even though the bottom surface was rough. There was no significant wavelength shift after an environmental test, which indicates that dense oxide films were formed at low-temperature with O2 cluster ion assisted deposition.