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Abstract: We have deposited germanium carbide (Ge1−x C x ) films on Si(100) substrate via radio-frequency (RF) reactive magnetron sputtering in a CH4/Ar mixture discharge, and explored the effects of carbon content (x) on the chemical bonding and hardness for the obtained films. We find that x significantly influences the chemical bonding, which leads to a pronounced change in the hardness of the film. To reveal the relationship between the chemical bonding and hardness, first-principles calculations have been carried out. It is shown that as x increases from 0 to 0.33, the fraction of sp3 C–Ge bonds in the film increases at the expense of Ge–Ge bonds, which promotes formation of a strong covalently bonded network, and thus enhances the hardness of the film. However, as x further increases from 0.33 to 0.59, the fraction of sp3 C–Ge bonds in the film gradually reduces, while that of sp3 C–H and graphite-like sp2 C–C bonds increases, which damages the compact network structure, resulting in a sharp decrease in the hardness. This investigation suggests that the medium x (0.17