Using three-dimensional topography data of a rough surface, a computer simulation was performed to investigate the effect of plastic deformation on normal loading and unloading cycles. A micromechanical model for elastic-plastic deformation was proposed and incorporated in the simulation. The model includes the effect of elastic recovery observed in laboratory indentation hardness tests. The results of the simulation show that hysteresis and permanent sets typically observed in laboratory experiments are caused by plastic deformation. Even if the physical properties of the contacting material are of hard rock and the change in surface roughness after the loading and unloading cycles is small, the amount of hysteresis and permanent sets is significantly large. Plastic deformation plays an important role in contact mechanics of rocks. ¿ American Geophysical Union 1994