Oxidized-low-density lipoprotein (ox-LDL), which is generated by LDL oxidation, is a risk factor of cardiovascular disease development. LDL has positive charges on the surface attributable to amino groups of lysine moieties on Apo B-100. Its positive charge is decreased by oxidation. For this paper, the topography and surface potential of LDL and ox-LDL are measured using atomic force microscopy and Kelvin probe force microscopy (KPFM). Although it is difficult to ascertain a difference between topographic images of LDL and ox-LDL, the KPFM images of ox-LDL differ from those obtained using LDL. The averaged potential of the surface decreases concomitantly with increasing oxidation time of the LDL. We demonstrate a method to evaluate the oxidation level of LDL using KPFM.