Sulfur doped DLC nanocomposite film was fabricated on n-type silicon substrates by the electrochemical method using the mixture of methanol and thiofuran as the precursor. The synthesis and microstructure of the composite film was investigated. Compared with pure carbon film, sulfur doping effectively enhanced carbon film graphitization, and reduced the surface roughness, confirmed by XPS, Raman and AFM. In particular, XPS analysis revealed sulfur atoms among amorphous carbon matrix was in the form of organosulfur compounds. Furthermore, a novel but feasible growth mechanism was proposed following the synergism of thermochemistry, plasma chemistry and electrochemistry processes.