Abstract: 48 surface samples from the northern part of South Yellow Sea were analyzed with traditional organic geochemical methods. Saturated hydrocarbons in the sediment organic matter were studied by GC and GC-MS. Results show that the organic matter had been degraded by the microbe, and the chromatogram of the saturated hydrocarbon shows certain amount of UCM, in a pattern of bimodal distribution, with predominate odd carbon number. The values of Pr/nC17 and Ph/nC18 are high. The biomarkers of terpane and sterane show that the organic matters are matured and began to generate hydrocarbon. Multidisciplinary study reveals that the organic matters in the seabed surface sediments of the northern South Yellow Sea are mainly coming from terrestrial high-rank plants and are well preserved, and the maturity of organic matter is rather high. All of these suggest a kind of exogenous hydrocarbon contamination.