Microstructural changes in yttria-stabilized zirconia with aging were investigated by X-ray diffraction, high resolution transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy under TEM, and electron diffraction. First, various microstructural changes reported in the literature as the causes of the decrease in conductivity were carefully examined. As a result, almost all of them proved not to be the direct cause. Therefore, the changes with aging were analyzed mainly by calling attention to a characteristically broad area that appears on the left shoulder of the X-ray diffraction peaks as well as the superlattice reflections and diffuse scattering seen on electron diffraction patterns, and the relationship between microstructural changes and conductivity was clarified. It is concluded that various phenomena due to aging at relatively lower temperatures are caused by short range ordering of oxygen ion vacancies, which occurs in order to relax the anisotropy of periodical lattice distortion. We also studied changes in microstructures occurring with changing dopant concentrations and concluded that the changes in microstructures responsible for the decrease in conductivity associated with increasing dopant concentration are caused by short range ordering of oxygen ion vacancies in order to relax the anisotropy in strain fields, similarly to those with aging.