Ti-B-N thin films with different B contents were deposited on Si (100) at room temperature, followed by vacuum annealed at 400, 600, 800 and 1000°C for 1 h respectively. Effect of boron content on thermal stability was investigated using X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy and nanoindentation measurements. The results indicated that incorporation of B into TiN produced a nanocomposite structure, which had a positive effect on microstructure stability. A high B content resulted in an elevated recrystallisation temperature. The hardness stability was not consistent with that of microstructure, and depended on phase configuration and composition. The films with a high as deposited hardness showed high hardness stability. Excessive or lack of amorphous phase decreased hardness stability. The residual stress value was decreased with increasing annealing temperature owing to recovery of amorphous matrix, and crystallisation of amorphous phase made its direction transform from compression to tension. © 2006 Institute of Materials, Minerals and Mining.