Highly charged ions (HCIs) have huge potential energy due to their high charge state. When a HCI reaches a solid surface, its potential energy is released immediately on the surface to cause a nano-scale defect. Thus, HCIs are expected to be useful for solid-surface modifications on the nano-seale. We investigate the defects on a highly oriented pyrolytic graphite (HOPG) surface induced by slow highly charged Arq+ ions with impact energy of 20-2000qeV with scanning probe microscopy (SPM). In order to clarify the role of kinetic and potential energies in surface modification, the nano-defects are characterized in lateral size and height corresponding to the kinetic energy and charge state of the HCIs. Both the potential energy and kinetic energy of the ions may influence the size of nano-defect. Since potential energy increases dramatically with increasing charge state, the potential energy effect is expected to be much larger than the kinetic energy effect in the case of extremely high charge states. This implies that pure surface modification on the nano-scale could be carried out by slow highly charged ions. The mean size of nano-defect region could also be controlled by selecting the charge state and kinetic energy of HCI.