Sea salt aerosol (SSA) is an important link between ocean and atmosphere. Surface-active organics constitute a major fraction of the organic film of SSA. It is believed that the changes in chemical composition and surface organization of the organic film have important influences on physical, chemical and optical properties of SSA, and then affect many atmospheric processes. A Langmuir trough was used in combination with infrared reflection absorptionspectroscopy (IRRAS) to detect the molecular arrangement of the surface film from macroscopic viewpoints as well as microscopic consideration. Focusing on the mixed monolayers of stearic acid/oleic acid (OA) and stearic acid/elaidic acid (EA) at the air-water interface, the intermolecular interactions between long-chain fatty acids at the SSA interface were investigated, and the effects of saturation degree and double bond configuration of organics on the surface properties of SSA were clarified. It is found that compared with EA with trans-double bond, the obstructive effect on the molecular ordering was found to be significantly remarkable for OA where the cis-double bond exists in the middle of molecular chain. Although the trans-double bond in the alkyl chain of EA also produces the obstruction, the degree is not as much as that caused by the cis-one.