Sulfur cycles in micro zones of sediments at a tidal flat was investigated using isotope fractionation (δ³⁴S) of sulfate, acid volatile sulfides (AVS), and pyrite. Aerobic oxidation is major driving force for the sulfur cycle in the sediments that located very close to the sediment surface (0.5 mm depth from the sediment surface) in the tidal flat. During this cycle isotopically light sulfate is accumulated. Shift of δ³⁴S values of the sulfate toward positive direction proceeds with reduction by sulfur reduction bacteria (SRB) under anoxic conditions at depths between 0.5 cm and 7 cm below the sediment surface: At the depth where the shells locate (15 cm below the sediment surface), the major sulfur compound is pyrite. The δ³⁴S values of the AVS are between those of sulfate and pyrite for sediment samples collected at both inside and outside the shells. Although sulfate δ³⁴S and pyrite δ³⁴S values are almost the same for the inside and outside of the shells, the difference of the AVS δ³⁴S values between the inside and outside is quite significant. Seasonal variation of δ³⁴S values was also observed for the AVS. Unknown sulfur oxidizing bacteria (SB), which oxidize AVS into sulfate accompanying significant sulfur isotope fractionation under anoxic conditions, may play the major role of the isotope fractionation.