In this study, high-temperature oxidation of single-crystal diamond and diamond films prepared by hot filament chemical vapor deposition (HF-CVD), were characterized using thermal analysis and high-temperature in-situ Raman analysis. The measurements were performed in various temperatures up to 1300°C in air and N2 atmospheres. The results indicate that the initial oxidization temperature of diamond film deposited at 700°C (D700 film) is ≈629°C, lower than those of diamond film deposited at 900°C (D900 film, ≈650°C) and single-crystalline diamond (≈674°C) in air. Oxidation rate of D700 film at high temperatures appeared to be the highest among the samples studied. A likely cause lies in the fact that, compared to their D900 sample, D700 diamond film contains a larger amount of non-diamond carbon and grain boundaries. However, D900 and D700 diamond films as well as single-crystalline diamond showed no detectable weight loss and oxidization when they were heated up to 1300°C in N2 atmosphere.