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To obtain vertically-oriented helical-peptides in the lipid Langmuir-Blodgett (LB) films, three types of helixforming "amphiphilic peptides" (APs) with different length of the hydrophobic domain were designed, and the orientation of the helices and the surface morphology of the AP/lipid mixed LB films were investigated. The structural models, which were consistent with the results obtained by different types of measurements, were revealed that a phase separation between the APs and lipids occurred in the mixed film and the helix orientation significantly depended on the length of the AP's hydrophobic domain. When the hydrophilic domain of AP consisted of three lysine residues, the hydrophobic domain consisting of 15 residues resulted in a uniform orientation of helical AP, whose helical axis tilted 27 • from the film normal, but the longer or shorter hydrophobic domain than the 15 residues resulted in a mixture of vertically and horizontally oriented APs. Thus, the balance between the hydrophobic and hydrophilic domains in the AP is important to control the orientation of the helical AP in the AP/lipid mixed LB film.