The relationship between the optical degradation and the morphology of hydrogenated amorphous silicon (a-Si:H) films was studied through the analysis of the annealing processes. The annealing of optically induced metastable dangling bonds can be explained in terms of the first and the second order reactions with dispersive hydrogen diffusion for the films deposited at lower and higher temperature, respectively. The activation energy of the annealing was estimated to be 0.8 eV with a small distribution in both cases. Optically enhanced annealing was first observed. It was found by DLTS and CT measurements that the density of hole trap located at Ev+0.6 eV was remarkably decreased by the light soaking, accompanied by the simultaneous increase of electron trap at Ec-0.7 ∼ Ec-0.9 eV due to the dangling bonds. These experimental facts support the view that the hole trap corresponds to the weak bonds, which convert to the metastable dangling bonds by light soaking.