Thin films of amorphous silicon-carbon alloy (a-Si1-xCx) were obtained by sputtering silicon and carbon target with argon in radio-frequency magnetron sputtering equipment. Both Si-C and C-C combinations form structures close to the randomly combined structure and the Si-Si combination at x > 0.5 shows a feature of the chemically ordered structure. The film at 0.6 < x < 0.7 is hard and shows a high energy gap, due to the sp(3) configuration in C-C combinations formed by the effect of surrounding tetragonal structure of silicon.