Ultrananocrystalline diamond (UNCD) films prepared by hot filament chemical vapor deposition (HFCVD) were annealed at 1000 degrees C in low degree vacuum under a pressure of 4000 Pa. The correlation between the mechanical and structural properties was investigated to understand the oxidization behavior of UNCD films after high temperature annealing. At the early stage of annealing (similar to 30 min), the amorphous carbon and graphite in grain boundaries are selectively oxidized firstly along the clusters' gaps, with the Young's modulus and hardness decreasing rapidly revealed by nanoindentation results. A special annealing time of similar to 30-60 min is found to exist as a turning point that the mechanical properties changing trend has a transition, because of the diamond grains starting to be oxidized. With the annealing time increasing to 180 min, the nanoindentation depth increases from similar to 70 to similar to 90 nm and the Young's modulus and hardness decrease more slowly with almost keeping constant of similar to 383 and similar to 35 GPa, respectively. X-ray photoelectron spectroscopy (XPS) results show that a steady 30-nm-thick oxidized layer has been formed on the top-surface and keeps a balance of the speed between films being oxidized and the carbon oxidation being broken down. (C) 2014 Elsevier BAT. All rights reserved.