Molecular dynamics simulations were conducted in order to study effects of mono-atomic scale steps on sliding surfaces on the dynamic behaviour of lubricant molecules under high pressure and shear. Hydrocarbons, including nhexane, cyclohexane and n-hexadecane, were assumed as lubricants. Simulations were made such that two layers of lubricant molecules were formed. It was found that steps on the surfaces had dramatic effects on interactions between surface atoms and lubricant molecules. Movements of lubricant molecules, and thus traction between the surfaces, were affected by interactions between the molecules and those between the molecules and the roughness structure, both of which depended on the molecular structure of the lubricant.