To explore the relationship between the chemical bonding and mechanical properties for germanium carbide (Ge1-xCx) films, the Ge1-xCx films are prepared via reactive magnetron sputtering in a mixture of CH4/Ar discharge, and their composition, chemical bonding and hardness were investigated as a function of substrate temperature (Ts). The results show that Ts remarkably influences the chemical bonding Of Ge1-xCx film, which results in a pronounced change in the film hardness. As Ts increases from ambient (60 ℃) to 500 ℃, the Ge content in the film gradually increases, which promotes forming sp3 C-Ge bends in the film at the expense of sp2C-C bonds. Furthermore, it is found that with increasing T, the fraction of C-H bends in Ge1-xCx film gradually decreases, which is attributed to an enhancement in the desorption rate of C-Hn (n =1, 2, 3) species decomposed from methane. The transition from graphite-like sp2 C-C to diamond-like sp3C-Ge bonds as well as the reduction in C-H bonds in the film with increasing Ts promotes forming the compact three-dimensional network structure, which significantly enhances the hardness of the film from 5.8 to 10. 1 GPa.