Studies were done on the phospholipid fatty acid composition of the most important aquatic animals flesh which represents the freshwater fish and marine fish. In these species, important differences were discovered in the quantitative composition of the omega-3 fatty acids. The present data give an evidence that the freshwater fish contains higher levels of arachidonic (AA, 20:4 n 4) and linoleic (18:2 n-6) acids than those in marine fish. However, in marine fish, n-3 fatty acids, particularly docosahexaenoic (DHA, 22:6 n-3) and eicosapentaenoic (EPA, 20:5 n-3) acids were the dominant unsaturated fatty acids, while the n-6 fatty acids (20:4 n 6, 18:2 n-6 and 22:5 n-6) acids were found in low levels. The ratio f n-3/n-6 and C18:1/ C18-2 were much higher in marine fish than those in freshwater fish. So we may concluded that the beneficial effect of fish oils especially marine fish is attributed mainly to the large amount of 22:6 n-3 and 20:5 n-3 fatty acids which reduce the risk of cardiovascular diseases, cancer and obseity.