To compare the thermal stabilities of a fluorinated amorphous-silicon (a-Si: F) and a deuterated one (a-Si: D), variations in the compositions of both a-Si: F and a-Si: D after annealing at temperatures of up to 600°C are experimentally studied. Among various compositions, the fluorine (F) content in a-Si: F and the deuterium (D) content in a-Si : D, in particular, are evaluated from the Rutherford backscattering measurement and the nuclear reaction measurement, respectively, after each annealing step. It is found that the a-Si: F is stable up to 600°C while the a-Si: D is not stable above 350°C.