This paper deals with the role of hcp-AIN on the thermal stability and hardness variation of Ti1-xAlxN thin films after annealing. In terns of phase formation, three types of coatings were considered: single-phase solid solution TiAlN, decomposed TiAlN+AlN without hep-AlN and nanocomposite TiAlN films with a significant fraction of hcp-AlN at the as-deposited state. The films were annealed under vacuum at 1000 degreesC from 30 min to 16 h and were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM) and nanoindentation. After annealing, the hardness of single-phase films was significantly decreased due to stress relaxation, grain growth and alteration of intercolumnar boundaries because of oxygen diffusion. In contrast, the hardness of nanocomposite films was first increased after 2 h of annealing and remained nearly constant for annealing times up to 16 h. The enhancement of hardness at initial stage of annealing is attributed to structural modification of hcp-wurtzite AlN and related volume reduction. The stability of hardness for long-term annealing is associated with nanograin multiphase structures preserving coherency stresses which are the sources of hardening in these coatings. For the decomposed samples, the general behaviour remains between those of single-phase and nanocomposite films. (C) 2004 Elsevier B.V. All rights reserved.