Using first-principles method, the frictional properties of diamond-like carbon (DLC) films containing 66.7 and 80 at.% S are well studied. S bulk-doped diamond induces graphitization of surficial carbon atoms, which could well explain the failure mechanism of DLC films with 80 at.% S under high vacuum. Observed from potential energy surfaces, S surface-doped diamond exhibits obviously low friction paths during sliding, which could account for the low friction and long lifetime of DLC films with 66.7 at.% S.