Molecular orientation of amphiphilic alpha-helical di-block type copolypeptide PLLysZ(25)-P(MLG(42)/LGA(18)) was controlled in a monolayer at an air/water interface by controlling the secondary structure of hydrophilic segment of the peptide. The orientation of the peptide was investigated by surface-area (pi-A) measurement, FT-IR reflection-absorption spectroscopy (RAS), and atomic force microscopy (AFM).