Thin films of amorphous germanium-carbon alloy (Ge1-xCx) were obtained by sputtering germanium and carbon targets with argon in radio-frequency magnetron sputtering equipment. The films with the same value of x : had some differences in structure depending on deposition conditions. For a radio-frequency power of 50 W, the film mainly exhibited Ge-C bonds and showed the maximum energy gap for each composition x. For higher values of radio-frequency power, however, the film included excess germanium clusters which gave a lower value in energy gap.