Several thin films of Ti-B-N with different N contents were deposited at 500 °C on Si (100) substrate by reactive unbalanced dc magnetron sputtering, followed by vacuum annealed at 600, 800 and 1000 °C, respectively. The effect of nitrogen content on microstructure, mechanical behaviors and thermal stability was investigated by X-ray diffraction (XRD), plan-view high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) and microindentation methods. The results indicated that nitrogen content had a great effect on phase configuration, microstructure and the corresponding mechanical behaviors. An amorphous (a-) Ti-TiB compound thin film which consists of Ti and TiB with a low hardness value was formed without nitrogen doped. Addition of small amount of nitrogen (10 at.% N in this study) produced nanocomposite nanocrystalline (nc-) Ti(N)/a-TiB thin films by way of activating phase segregation. Increasing nitrogen content accelerated further segregation of phase, accompanied with formation of nanocomposite nc-TiN/a-TiB thin films and fast increase of microhardness value. A maximum microhardness value of about 52 GPa was obtained at TiBN. Further increase of N content accelerated formation of BN bonding, followed by formation of nc-TiN/a-TiB/a-BN thin films and fast decrease of microhardness value. The residual stress was not consistent with the microhardness, and showed linear increase with increasing N content. The thermal stability of Ti-B-N thin films strongly depended on microstructure and phase configuration. A high thermal stability usually occurred at an optimum composition which had a high microhardness value. © 2006 Elsevier B.V. All rights reserved.