The microstructure and nanomechanical behavior of diamond like carbon (DLC) films has been investigated at high temperature annealing. The DLC films with 250 nm thickness were deposited on the crystalline silicon (c-Si) substrate using ultra-high-vacuum ion beam sputtering and post thermal annealing at high temperature up to 900 °C for 0.5-J.5 hours. Raman spectra showed that the as-deposited DLC film contained tetrahedral amorphous carbon (ta-C) state and presence of increased Sp2 bonds after annealing at 900 °C. GIXRD spectra obtained for the DLC films indicated the absence of diffraction peaks at room temperature (RT) and varied vacuum annealing conditions. It indicates that the above DLC films are still amorphous at high thermal stability despite the change of bonding behaviour from primary Sp3 to more Sp2 bonding. The nanohardness and elastic modulus of the DLC films measured by continuous stiffness measurement technique in the nanoindentation decreased significantly from 29.6 and 35J.2 GPa at RT to 17.5 and 150 GPa at 900 °C, respectively which has been attributed to the graphitization of DLC films at high temperature.