Abstract: The compositions and relative abundances of fatty acids in samples of two mangrove species (Kandelia obovata and Aegiceras corniculatum) and two saltmarsh species (Spartina alterniflora andCyperus malaccensis) in different stages of the matured, senescent and decomposed were compared in Jiulong River Estuary in South China. A total of 20 fatty acids were measured, nine of which were commonly found in all samples. The most abundant saturated fatty acid was palmic acid (C16:0, 21.76%-39.55%), followed by octadecoic acid (C18:0, 4.98%-29.88%). Unsaturated fatty acid mainly comprised oleic acid (2.04%-12.90%), linolic acid (8.25%-24.69%) and 9-eicosenoic acid (1.64%-40.45%). The mangrove and salt marsh species had a different fatty acid composition in the samples and C30:0 was the biomarker of mangrove species. There was also a difference in the fatty acid composition of the senescent and decomposed samples relative to that of mature samples. C20:1n9 was higher in mature samples than in senescent and decomposed samples. Salt marsh plants decreased more rapid in the senescent samples than those of mangrove samples. The results suggest that the senescent and decomposed samples of the four plants have a much lower unsaturated fatty acid level and the essential fatty acid (C18:2n6) in the samples does not increased proportionally during the senescence and decomposition of the plant tissues.