An ion beam sputtering system was used for the deposition of indium-tin-oxide (ITO) films at low temperatures (below 200 degreesC). The electrical and optical properties and the microstructure were highly dependent on the growth temperature, the oxygen partial pressure and the ion beam energy. A reasonable resistivity (3.5 x 10(-4) Omega cm) was measured in the films deposited by Ar ion sputtering at as low as 50 degreesC. In the films by Ar ion sputtering, the lowest resistivity was 1.5 x 10(-4) Omega cm at 100 degreesC. Oxygen addition to the sputtering gas increased the resistivity, especially at low substrate temperatures. The addition of oxygen to the sputtering gas changed the microstructure from 'domain' (sub-grain) structure at 100 degreesC to 'grain' structure. The oxygen addition induced the change in O/In ratios. The him composition also depended on the ion beam energy. The optical transmittance higher than 80% in the visible range was measured in the films deposited at above 100 degreesC. The optical band gap calculated from the transmittance spectra was approximately 4.2 eV.