Oxidation kinetics of diamond films synthesized by plasma-assisted chemical vapor deposition (CVD) in flowing oxygen were evaluated using thermal gravimetric analysis. The oxidation rates of diamond films, measured in the 500 to 750 °C range, were significantly lower than natural diamond, which was lower than grafoil and pyrolytic forms of graphite. The mechanisms of oxidation in the CVD diamond films and natural diamond were explored by scanning electron microscopy and Auger electron spectroscopic examination of localized regions. In CVD films, oxidation occurred preferentially at grain boundaries, local defects, and diamond-like carbon containing regions during the early stages of oxidation process. The results suggest that preferred orientation of diamond crystallites in the CVD film plays a major role on its oxidation behavior.