It is well known that a TiC layer can be formed and should act as a buffer layer in diamond films deposited on Ti alloy. Through our cross-sectional investigation in HRTEM, a thin layer (20-30 nm) was first identified between the outermost diamond film and the inner reactive TiC layer adjacent to the substrate. This layer consists of numerous crystalline nanoparticles with grain sizes of 5-20 nm. Through electron nanodiffraction patterns combined with EDS and EELS analysis, these nanoparticles can be identified as a TiC1-xOx phase with a similar structure to cubic TiC. Besides, C atoms and O atoms in TiC1-xOx randomly occupy the vacancies of C in TiC. The thickness of this TiC1-xOx layer does not change significantly with increasing deposition time, and the diamond phase directly nucleates and grows on it. (C) 2013 Elsevier B.V. All rights reserved.