The surface parameter of protonated surfactin molecules and the structural properties of the protonated surfactin monolayer adsorbed at the air/water interface have been studied by molecular dynamics simulation. The simulation was performed at 293 K and the interfacial concentration of surfactin was set in a range of 0.70-2.20 nm(2) molecule(-1). The results show that the interfacial concentration greatly affects the molecular orientation of surfactin, the structure of the peptide ring backbone and the spatial arrangement of the surfactin monolayer. The peptide ring backbone of the surfactin molecule exhibits a structural flexibility, and a more packed structure is adopted at higher interfacial concentration. The hydrophobic contacts between surfactin molecules and the stability of the secondary structures, beta-turn structure in Leu2 -> Asp5 and the beta-sheet domains, are enhanced when the surfactin molecules are in a very packed situation.