Hydrogenated amorphous silicon carbide (a-SiC:H) thin film deposition was carried out by a remote plasma enhanced chemical vapor deposition (REPECVD) method. An organosilicon compound, hexamethyldisilane (HMDS) was used as the source material. We put emphasis on how plasma excited neutral radicals react with source material. It was found that nitrogen radicals react with the precursors and form stable Si-N bonds easily if nitrogen is used and excited by plasma. Thus, the film formation steps are simple and a high-speed deposition can be obtained, but the film shows low quality due to a large amount of incorporated nitrogen. Argon, hydrogen or their mixture is suitable for high quality film deposition but shows low deposition rate due to a rather complex reaction step. Hydrogen radical plays an important role in REPECVD, it can enhance the chemical reaction in the vapor phase, lead to deposition rate increasing, and also reduce hydrogen and methyl group content of the film.