Thin films of a-SiC:H and a-SiC:H(F) deposited under high ion bombardment at room temperature by plasma-enhanced chemical vapor deposition were annealed at temperatures ranging from 300 to 600 degrees C. The effects of the annealing on stress, bonding structures, hydrogen content and optical properties were studied. Both films tend to become more cross-linked as the annealing temperature is increased, but beyond 500 degrees C the film appears to become oxidized. The hydrogen content of a-SiC:H film starts to decrease when the annealing temperature exceeds 400 degrees C. Upon annealing, the optical transparency in the visible region improved slightly upon annealing with a-SiC:H film, whereas the opposite behavior is observed with a-SiC:H(F) film. Strikingly different behaviors were observed in the film stress. For a-SiC:H film, the stress decreases progressively as the annealing temperature is increased and becomes zero at 600 degrees C. For a-SiC:H(F) film, the stress becomes zero at a relatively low annealing temperature of 200 degrees C, beyond which the stress changes from compressive to tensile.