Abstract: The lipid biomarkers of fatty acids in Site4B sediments from Shenhu Area, Northern South China Sea are studied in this paper and the sources of branched fatty acids and monounsaturated fatty acids are discussed. The results reveal that i/a-C_15:0, i/a-C_17:0, 16:1ω5 and 18:1ω9 are derived from sulfate reducing bacteria (SRB), while 16:1ω7t/c and 18:1ω7 are originated from sulfur-oxidizing bacteria (SOB). The distribution of SRB and SOB may be related with the process that sulfate was reduced to sulfide, and then sulfide oxidized to sulfate and element of sulfur, and at last elemental sulfur was disproportionated to sulfide and sulfate. In this process, SRB dominated the sulfur cycle system in the sediments. The increasing biomass of SRB and SOB at depths of 95~97 cm is related with diapire structure around Site4B, which carries a great amount of nutrient fluid.