The nanocomposite 'nc–TiN/a–BN' as a representation of the family of superhard nitride–based nanocomposites, which is a nanocomposite thin film material, exhibits a significant hardness enhancement as compared with the pure constituents. In this paper, first–principles calculations were performed to investigate the role of interfaces in the nanocomposite 'nc–TiN/a–BN', to which less attention has been paid up to now. In order to determine theoretically the stable interface configuration in 'nc–TiN/a–BN', 16 possible theoretical TiN(111)/BN/TiN(111) sandwich interface cnfigurations have been constructed based on the stucture characteristic of 'nc–TiN/a–BN'. It is found in this calculation that the mst favorable interface configuration istop–top–BN, which is closely related to each B atom covalently bonding to its tetahedrally coordinated N atoms in it. ts electronic structure is calculated. The calculated results show that the bnds at the interface in 'top–top–BN'configuration are covalent. Its interface bonding strength is higher than that between two 111 crystalline planes in slab TiN or bulk TiN.