Silicon, a potential anode material for all-solid-state batteries, has the highest theoretical capacity during electrochemical reactions, but is vulnerable to structural expansion and irreversible capacity. Amorphous Si/C composite thin films have received attention as promising materials to address these obstacles. The amorphous Si/C composite thin films were synthesized by radio frequency magnetron co-sputtering, which easily fabricated uniformly dispersed nanocomposites, prevented the agglomeration of silicon particles, and more effectively resisted volume expansion than traditional techniques. Electrochemical evaluation of the Si37C63 specimen showed that it had a high 1st cycle capacity (∼6180 mAh cm−3) and a capacity retention of ∼96% from 100th to 200th cycle. These studies show that the stresses induced by volume expansion during long term cycling tests were reduced by the buffering effects of the carbon content. The homogeneously dispersed amorphous Si/C composite thin films were found to be promising anode materials for all-solid-state batteries.