Changes in local structures with aging in yttria-stabilized zirconia were examined by extended x-ray absorption fine structure (EXAFS) and internal friction measurement. An analysis of EXAFS shows that a decrease in the first nearest neighbor coordination number of Zr ions, which means short range ordering of oxygen ion vacancies, a decrease in distance between a Zr ion and its first nearest atoms, and an increase in distance between a Zr ion and its second nearest atoms occurred in aged 8YSZ. Aging caused a noticeable decrease in the relaxation peak of internal friction in 8YSZ. These phenomena are attributed to the short range ordering of oxygen ion vacancies around a Zr ion that results from relaxation in the anisotropy of periodical lattice distortion. Therefore, it is concluded that the trapping of oxygen ion vacancies at Zr ions in order to relax such distortion is responsible for a decrease in conductivity with aging. The relaxation peak of internal friction before aging changed with increasing dopant concentrations, and the behavior of this change from 8YSZ to 10YSZ was very similar to that when 8YSZ was aged, and that from 10YSZ to 15YSZ was similar to that when a single crystal of 10YSZ was aged. This marked similarity indicates that the changes in crystal structure which cause the decrease in conductivity with increasing dopant concentration are caused by the short range ordering of oxygen ion vacancies brought about by relaxation of the distortion, similar to those when conductivity decreases with aging.