HD formation reaction was investigated in H2 plasma over deuterium-incorporating hydrogenated amorphous silicon (a-Si:H:D). The activation energy of the reaction was determined to be 2.4±0.7 kcal mol−1, which was consistent with the abstraction reaction of hydrogen from some silane derivatives by hydrogen atoms. The HD formation reaction could be explained by the hydrogen abstraction reaction. The hydrogen abstraction from a-Si:H surface by H atoms is thought to be one of the elementary growing processes of an a-Si:H in a SiH4-H2 plasma.