Amorphous SiC thin films were deposited from a mixture of silane and methane gas with the separate hydrogen and Argon dilution by using the plasma enhanced chemical vapor deposition process. The effects of different dilution gas on the chemical composition, bonding states, microstructure and optical property of the films were studied and compared. For the films with H2 dilution, the higher hydrogen content passivated more Si dangling bonds so that the formation of nanocrystalline silicon was inhibited and the films were kept in amorphous state. The amorphous nature along with the higher carbon content of the films led to the larger optical band gap E(opt), of 2.63 eV. In contrast, for the films with Ar dilution, they presented a relative higher deposition rate due to the rapid decomposition of silane accelerated by Ar. The Si-Si bonds were formed and they had a higher tendency to precipitate silicon quantum dots.