The behaviors of polymer monolayers on a water surface were investigated by the measurement of surface pressure-area isotherms. A good correlation of the collapse pressure of the monolayer with the hydropathy scales of the natural amino acid rather than the hydrophobicity scales was observed. It is concluded that the amino acid residue in the polymer monolayer on the water surface exists in a similar molecular environment to that of the polypeptides. The hydropathy scale for the amino acid residue can be obtained experimentally from the collapse pressure of the corresponding monolayer, and that for unnatural amino acid residues also was examined. Moreover, the dependence of the hydrophobic character of the amino acid on temperature is also investigated by the change in the collapse pressure of the amino acid derivative polymer monolayers.