The saturated fatty acids(arachidic acid, stearic acid, and palmitic acid) were repeatedly applied on water surface, and their ${pi}-A$ isotherms and Maxwell displacement current(MDC) were measured to identify the effect of the number of carbons of fatty acids on the properties of Langmuir films. The saturated fatty which contain higher carbon number showed higher MDC peak, and the less carbon it contains, the higher surface pressure they show. It was found the maximum of MDC peaks was as the molecular area just before the surface pressure increases in compression cycles of saturated fatty acids.