Plentiful bacteria are observed in anoxic sediments, where inorganic sulfur compounds can be reduced, oxidized, or/and disproportionately reacted by bacterial action and the bacteria acquire metabolic energy through those reactions. Bacterial reduction and disproportionation bring a significant isotope fractionation. For bacterial sulfate reduction (BSR), in general, the isotope fractionation between sulfate and H2S ranges between 4‰ and 46‰, with an average of 21‰, but the isotope fractionation is small (less than 5‰) and negligible for bacterial sulfide oxidation. The bacterial disproportionation of the sulfur intermediates, such as S0, S2O2-3 and SO2-3, results in isotope fractionations of 7‰～11‰. The information of sulfur isotopic fractionation from cultural experiments and modern ocean investigations has been used to interpret sedimentary record and reconstruct chemical evolution of ocean in the Earth history.