Annealing in vacuum at temperatures up to 820 o C was used to study the thermal stability of mechanical properties of magnetron-sputtered thick (approx. 1.5 μm) a-C films. A predominance of sp 2 bonds was characteristic for all these films. The microhardness, internal stress, electron diffraction, Raman and optical spectra of three sets of films with different initial microhardness (H~50, 20 and 10 GPa, respectively) were compared. Annealing of the hardest film up to 500 o C led to an increase in microhardness accompanied by a decrease in internal stress. Internal stress did not relax completely for hard films, even after annealing up to 820 o C, and the microhardness remained rather high (~40 GPa). Both the high internal stress and the specific film nanostructure are responsible for the high microhardness of these sp 2 -bonded films.