The many biogenic and nonbiogenic pyrites in the 607cm long core H-106 taken from 70m water depth in the central South Yellow Sea were composed of numerous tiny framboids consisting of countless tiny octahedral crystals. Electron microprobe analysis showed the pyrites were mainly composed of sulfur (50.59% in average) and iron (47.94% in average). Lighter sulfur dominate the sulfur isotope composition of pyrite. The values of sulfur isotopes range from-39.5 to-28.9 per mil (average of-36.0 per mil). The sulfur isotope fractionation coefficient between pyrite and sulfate in seawater is from 1.0510 to 1.0626. There was little variation of sulfur isotopic composition with core depth. The research showed pyrite formed in the sediments slowly deposited in the study area where average bottom water temperature in 24 years is 8.06℃ and average concentration of organic matter and iron in the sediments are 2.14% and 4.6%, respectively. Sulfur isotopic composition and other evidence showed the. pyrite here was formed slowly(about 10 years).