In order to understand the mechanism of the self-organization of surfactant molecules, we measured the interlayer spacings of lamellar and hexagonal liquid crystals as a function of the polyoxyethylene-chain length in water-polyoxyethylene alkyl ether systems. From these data, the effective cross sectional area per one surfactant molecule at the hydrophobic interface, αs, is calculated. The αs is almost independent of the hydrocarbon-chain length of surfactant, but dependent on the EO-chain length, n, and the shape of aggregates in liquid crystals. It is considered that the αs is determined by the balance between the cohesive force (the interfacial tension) of the hydrocarbon chain and the hydration force of the EO chain. Taking into account of the steric hindrance of hydrocarbon chain at the interface (the excluded area=0.20nm2 for lamellar liquid crystal) and the curvature effect of surfactant molecular layers, a new theory is proposed to calculate the αs for the rod, the spherical and the layer aggregates. At a particular EO-chain length, the aggregate to minimize the cross-sectional area is produced. As a result, when the EO chain of surfactant is short, a lamellar structure is most stable. With increasing the EO chain, the curved surface is more stable. Based on this theory, we also discuss about the change in the occupied surface area per one surfactant molecule at water-air interface in the aqueous surfactant solutions. Different from the self-organization in water, the occupied surface area at air-water interface obeys the equation for the flat surface even in the very long EO-chain surfactant because the curvature effect does not appear in this case.