A series of samples with different Si concentration were prepared by adjusting the numbers of Si-doped graphite targets and pure graphite targets. It was found that the stress in the thin films decreased from 4.5GPa to 3.1GPa when the Si concentration reached 6.7at.%，but the hardness kept constant at about 3600Hv，almost the same as of un-doped thin films，and the friction coefficient of thin films kept constant at about 0.15. As the Si concentration in the thin film kept on increasing，the concentration of C-Si bond will increase，leading to the decrease of hardness and stress and the increase of friction coefficient.