Abstract: Reported in this paper are the composition and distribution of alkyl cyclopentanes, alkyl cyclohexanes, linear alkylbenzenes and phenylalkanes in the seep carbonate from AC 645 (Sample AC-E) at the lower slope of Gulf of Mexico, where the water depth is 2 200 m. The distribution of alkyl cyclopentanes and alkyl cyclohexanes is similar to normal alkanes, which are dominated by low molecular weight compounds and maximized at C₁₄,C₁₅, respectively, suggesting that the alkyl cyclopentanes and alkyl cyclohexanes are mainly originated from bacteria and phytoplankton. The carbon number of alkylbenzenes are C₁₃-C₂₃, obviously dominated by even-numbered carbon and maximized at C₁₆, which are probably originated from halophilic bacteria or bacterium or sulfate-reducing bacteria in seep site, and the three group phenylalkanes are likely originated from blue green microalgae and planktonic organisms.