Abstract: The film pressure vs. area curves of the surface films of Triton X-100 and Triton X-305 have been determined on a 46.6% NANO_3 substrates at 25 ℃. The results suggest that the area per molecule on the surface at constant film pressure is a linear function of ethylene oxide chain length. It is proposed that the amphipathic molecule orients at the air/water interface with the hydrophobic group above the interface and the ethylene oxide chain partly lies in the interface and partly penetrates the water phase. Although the fraction of the number of ethylene oxide unit lain in the interface, f, increase progressively with film pressure, is independent of n, the ethylene oxide chain length. With this model, the experimental results can be explained.