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Molecular dynamics (MD) simulations were performed to investigate the geometric effect of nanostructured substrates on adsorptive behavior of potassium stearate monolayers. A typical protective coating, diamond-like carbon (DLC), was employed as adsorptive substrates. For the substrates with square grooves, complete penetration of molecules can only occur under large width and small depth. The occurrence of partial penetration would make monolayer atoms away from substrate surfaces and therefore induce weak substrate-molecules interactions. Compared with the square groove bottoms, monolayers are much more strongly adsorbed to the groove concave corners. For substrates with triangle and semi-circle grooves, complete penetrations are achieved in both cases and thus monolayer atoms can interact with the whole groove surface. The effect of substrate morphology on monolayer adsorption would be sufficiently weakened by the decrease of geometric size, and the surface adsorptivity of semi-circle grooves with large radius is similar to that of the smooth surface. The results indicate that the substrates with triangle grooves have better adsorptivity than other morphologies. • Size effect of nanostructured diamond-like carbon substrates on adsorption of potassium stearate monolayers was studied using MD simulation. • Geometric size of surface morphologies and amorphous atomic arrangement will significantly influence the molecular adsorption. • The monolayer-to-substrate adhesion at concave corners is much stronger than other positions within grooves. • Complete penetration of monolayers is achieved in all the morphology sizes for triangle grooves and semi-circle grooves. [ABSTRACT FROM AUTHOR]