Impact of micron-scale particles onto surfaces are encountered in numerous surface treatment methods such as shot peening and abrasive machining. Moreover, working parts exposed to particulate environments can suffer from excessive erosion due to particle impact. In such processes, properties of the surface before and after processing, as well as particle impact conditions significantly influence the final outcome. In order to explore the effect of surface properties on the particle impact process, normal impact of micron-scale copper particles onto a rough copper surface in the 25-150 m/s velocity range is investigated using the finite element method. Surface roughness is generated numerically and incorporated into the finite element model. Isotropic hardening with strain rate effects and thermal softening due to plastic heat dissipation are included in the model. The effect of surface roughness on the mechanics of impact is investigated, as well as the effects of impact velocity, particle size and number of impacts. Impacts on peaks and valleys cause response similar to oblique impact, affecting the rebound behavior of the particle. Surface smoothening upon impact is achieved by crushing the surface peaks; however, collapse of adjacent peaks can provoke stress concentrations and initiate crack formation. Influence of surface roughness on the aftermath of particle impacts decreases with increasing particle size and impact velocity. In general, it is concluded that the effect of surface roughness should be taken into account for low velocity impacts where only the surface peaks deform, or for small particles with size comparable to the surface roughness.