Interfacial behavior of surfactin methyl ester derivatives at the n-decane/water interface at low surface coverage has been studied by molecular dynamics simulation. Molecular orientations, structural variability of the peptide ring backbones, interfacial molecular areas, and the motion activities of surfactin derivatives have been determined. The simulations show that surfactin monomethyl ester stands vertically at the oil/water interface compared with surfactin molecule. The aliphatic chains tilt at the interface and can fold back to interact with the hydrophobic amino acid residues within the same molecule. Amino acid residues that the aliphatic chains are favorable to interact with are different between surfactin derivatives. The peptide ring backbones of surfactin and surfactin derivatives expand at the interface. Interfacial molecular areas of surfactin derivatives are all about 110 A(2). Translational and rotational motions of surfactin derivatives are limited at the interface, and the motion activities increase with the hydrophobic character of the peptide moiety.