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A group of 2,6,10,15,19-pentamethylicosenes (PMI△) containing 1-5 unsaturated double bonds has been identified in the sea floor sediments from the Gulf of Mexico at the Green Canyon 238 site.These PMI△ compounds are distributed between nC22 and nC24 on the mass chromatogram of aliphatic fraction. Their δ13C values are very much depleted in 13C and in the range of -86.7‰ to -115.5‰, whereas the δ13C values of companion n-alkanes range from -28.4‰ to -34.6‰. These unsaturated PMI△ compounds are typical biomarkers derived from the anaerobic oxidation of methane mediated by methane-oxidizing archaeal bacteria and indicative of the gas seeps or even the occurrence of gas hydrates in the deep sea sediments.