Lipid oxidation in aqueous systems is active area of research because lipids are present in aqueous medium in many food systems and in biological systems. Oxidative stability of polyunsaturated lipids generally decreases with increasing the degree of unsaturation, however, the stability in an aqueous solution is quite different from that in bulk phase or in organic solvent. In aqueous micelles, the oxidative stability of lipids increases with increasing the degree of unsaturation, and in emulsion, lipid oxidation is greatly affected by the properties of interface that consists of emulsifier and lipid. What is the reason for this characteristic behavior of the lipids in aqueous systems? NMR analysis gives useful information that more unsaturated fatty acids have higher flexibility of acyl chain conformation in aqueous systems. This flexibility brings about its high water permeability. The presence of water molecule near acyl moieties decreases the density of bis-allylic positions, where hydrogen radical abstraction occurs at the start of lipid oxidation; therefore, more unsaturated fatty acids is oxidatively more stable than less unsaturated fatty acids in aqueous micelles. Polyunsaturated fatty acyl moieties show the same behavior in other aqueous circumstances such as liposomes and cellular membrane to result the different oxidative stability from bulk phase and organic solvent system.