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Several nc-TiN/a-TiB2 thin films comprised of nanocrystalline (nc-) TiN and amorphous (a-) TiB2phases were deposited on Si(100)at room temperature by reactive unbalanced dc magnetron sputtering, followed by vacuum annealed at 400, 600, 800 and 1000℃ for 1h, respectively. Effects of B content on microstructure, mechanical behaviors and thermal microstructure stability have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and nanoindentation measurements. The results indicated that B addition greatly affected both microstructure and mechanieal behavior of nc-TiN/a-TiB2 thin films. With increasing B content the grain size decreased. A maximum hardness value of about 33GPa was obtained at B content of about 19at. %. The improved mechanical properties of nc-TiN/a-TiB2films with the addition of B into TiN were attributed to their densified microstructure with development of fine grain size. Only addition of sufficient B could restrain grain growth during annealing. High B content resulted in high microstructure stability. The crystallization of amorphous matrix occurred at about 800℃, forming TiB or TiB2 crystallite, depending on B content. Before that no change in bonding configuration was found.