The role of oxygen-radical (O-radical) in oxidation of titanium (Ti) by O-radical beam assisted evaporation was studied by in situ measurement of the deposition rate using a quartz crystal microbalance. Ti was evaporated simultaneously with oxygen molecule (O-2) and with O-radical. The deposition rates of the films deposited with O-radical were faster than those of the films deposited with O-2. This difference in the deposition rate resulted in the production of stoichiometric titanium oxides (TiO2) when using O-radical and suboxides when using O-2. In conclusion, it was clear that O-radical plays an important role in the effective production of stoichiometric TiO2. (C) 2003 Elsevier Science B.V. All rights reserved.