Nanometer-scale stripe pattern in the monolayer composed of alpha -helical copolypeptide was characterized with atomic force microscope (AFM). This monolayer was formed with the copolypeptide composed of poly(epsilon -benzyloxycarbonyl L-lysine)(x)-poly[(gamma -methyl L-glutamate)(y-z)/(L-glutamic acid)(z)] (PLLZ(x)-P(MLG(y-z)/LGA(z))). AFM image of Langmuir-Blodgett (LB) film of this copolypeptide showed a nanometer-scale stripe pattern. This pattern seems to be based on the nanophase-separated structure in the copolypeptide monolayer at air/water interface.