Ruthenium thin films were prepared by chemical vapor deposition using a liquid bis(ethylcyclopentadienyl)ruthenium [Ru(EtCp)(2)] precursor in an oxidizing atmosphere, and their growth mechanism is explained in terms of oxygen ratio. Before growth, a long incubation time exists and is shortened by increasing growth temperature and oxygen ratio. Following the incubation time, the Ru film grows linearly with increasing deposition time, and the rate-determining process dominating the precursor decomposition changes according to O-2/(Ar+O-2) ratio. At a low O-2/(Ar+O-2) ratio of 1.3%, at which film thickness increases in proportion to logarithmic ratio of O-2/(Ar+O-2), thermal decomposition with an activation energy of 1.9 eV dominates the growth process. On the other hand, at a high O-2/(Ar+O-2) ratio of 25%, at which sufficient oxygen gas is supplied and film thickness does not depend on O-2/(Ar+O-2) ratio, oxidative decomposition becomes dominant and activation energy decreases to 0.4 eV. We propose that the reaction-limited process results from the oxygen-supply-limited process, which is based on dissociative chemisorption of oxygen on the growing Ru film surface.