Germanium carbon (Ge1-xCx) thin films were deposited by ion beam sputtering deposition of Ge target in a CH4/Ar discharge. The surface morphology, chemical structure, infrared optical and mechanical properties of the Ge1-xCx films were investigated by atomic force microscopy (AFM), Raman spectroscopy, X-ray photoelectron spectroscopy(XPS), Fourier transform infrared (FTIR) spectroscopy and nano-indentation, respectively. The relationship between ion beam voltage and film properties was discussed. The results show that the surface roughness decreases with increasing of the ion beam voltage. The film deposited at a higher voltage has lower carbon content and higher fraction of Ge-C bonds. The film has excellent infrared optical and mechanical properties. The films show good transparent over a wide range. Due to the content of the carbon decreasing with increasing of voltage, the refractive index increases obviously as the ion beam voltage increases from 300 V to 800 V. The hardness of the film is above 8 GPa. With the Ge-C bonds instead of the C-C bonds and C-Hn bonds, the hardness of the film increases with increasing ion beam voltage.