Unsaturated fatty acids play an important role in vivo not only as energy sources but also as main components of biomembranes (1-3). It is well known that fluidity of biomembrane is affected by its characteristic dynamic structure and is dependent on the unsaturated fatty acid, oleic acid (4-8). Oleic acid, which has a double bond at C9 position in its molecule and freely-rotating, adjacent methylene groups, is one of the representative cismonoene unsaturated fatty acids which are able to bend comparatively freely. Until recently the purity of available oleic acid is low, and there has been restriction in its study for basic physical properties as well as its application for a speciality chemical field such as pharmaceuticals, cosmetics and biosciences. In recent years however, a product where purity is 99% or greater is constantly available and there have been many reports on the analysis of its basic physical properties (9-13) as well as the basic physical properties of nonionic surfactants which are its derivatives (14-16) and its use as an enhancer of drug penetration through the skin (17-20). These studies have shown that oleic acid has a big space-filling volume (21,22) based on cis-type double 325 JOS