Ta-Si-N thin films were potentially applied as diffusion barriers for Cu interconnections. However, the thermal stability of Ta-Si-N is related to the composition and annealing methods. In this paper, we have investigated the effect of highvacuum furnace annealing and vacuum rapid thermal annealing (RTA) on the microstructure and morphology of different nanostructured Ta-Si-N thin films fabricated by reactive cosputtering at varied Ta and Si powers and nitrogen flow ratio (FN2%= FN2/(FN2+FAr) × 100%). As Si is added to the Ta-N compound to form Ta-Si-N, the microstructure becomes nanocrystalline grains embedded in an amorphous matrix i.e. amorphous-like microstructure, which is also affected by the nitrogen flow ratio. Amorphous-like Ta-Si-N films obtained at small 3-6 FN2% had smoother morphology and lower resistivity compared to the polycrystalline film at high 20 FN2%. The thermal stability of Ta-Si-N films increases with the Si/Ta ratio and magnitude of vacuum. Higher vacuum furnace annealing at 5 × 10-5 Torr may make both amorphous-like and polycrystalline Ta-Si-N films enduring higher temperature up to 900 °C for a longer time of 1 h while the higher pressure RTA at 2 × 10-2 Torr make Ta-Si-N films transform of phase and morphology at 750-900 °C just in 1 min. The increase of Si/Ta ratio may also increases the stability of Ta-Si-N films.