We have conducted a comparative study on the deposition of diamond thin film on uncoated Al₂O₃, diamond-like carbon (DLC, grown by pulsed laser deposition) film-coated Al₂O₃, and Q-carbon (fabricated by nanosecond pulsed laser annealing)-coated Al₂O₃ substrates by hot filament chemical vapor deposition (HFCVD). Scanning electron microscopy shows that the continuous large-area diamond thin film can be grown on the Q-carbon/Al₂O₃ substrate, whereas the deposition of diamond on DLC/Al₂O₃ or uncoated Al₂O₃ substrates gives cluster-like/patches of discontinuous diamond thin film formation. Raman spectroscopy of the diamond on Q-carbon/Al₂O₃ shows a considerably small residual stress (1.7 GPa) compared to the diamond on DLC/Al₂O₃ (3.1 GPa) or diamond on the uncoated Al₂O₃ (3.9 GPa) substrate. The nondiamond content in the diamond crystals on Q-carbon-coated Al₂O₃ is found to be only 0.12%, which is about 2.5 and 3.5 times less than that of diamond on DLC-coated Al₂O₃ and uncoated Al₂O₃, respectively. Consistent with the Raman analysis, the X-ray diffraction analysis shows the crystalline growth of the HFCVD diamond on Q-carbon/Al₂O₃ with relatively small in-plane stress compared to the diamond film on the other two systems. We propose that the high density of diamond tetrahedron in the Q-carbon structure provides a good platform for diamond growth with a very high nucleation density over 10⁹ cm^–2 and can act as a seed layer for diamond growth without cracking or delamination over the surface of the substrate. The optically transparent high-quality diamond film on Q-carbon-coated Al₂O₃ showed better adhesion compared to that on DLC-coated Al₂O₃ and uncoated Al₂O₃, as measured by a simple scotch tape test. These results open a path toward the fabrication of large-area high-quality diamond films on an optically transparent substrate for commercial applications.