Ti-B-C-N nanocomposite coatings with different C quantities are deposited on Si(100) and high speed steel (W18Cr4V) substrates by the closed-field unbalanced reactive magnetron sputtering in the mixture of argon, nitrogen and acetylene gases. The microstructures of Ti-B-C-N nanocomposite coatings are characterized by X-ray diffraction and high-resolution transmission electron microscopy; while the nanohardness and elastic modulus values are measured by the nano-indention method. The results indicate that in the studied composition range, the deposited Ti-B-C-N nanocomposite coatings are found still only in the TiN base nanocrystalline. When the C2H2 flux is small, adding C can promote crystallization of Ti-B-C-N nanocomposite coatings, and the grain can be increased to improve the mechanical properties, when the grain size of about 6 nm (C2H2 flux rate 2 cm3/min), hardness, elastic modulus and fracture toughness of Ti-B-C-N nanocomposite coatings achieve the maximum, respectively, 35.7 GPa, 363.1 GPa and 2.46 MPa m1/2; the further increase of the C content of Ti-B-C-N nanocomposite coating can reduce mechanical properties of coating dramatically.