We report the formation of anomalous defect clusters in yttria stabilized zirconia (YSZ: 13 mol% Y2O3-ZrO2), which are a primary candidate for inert matrix fuels. Electron irradiation with 100 to 1000 keV is found to induce characteristic defect clusters. The defect clusters show strong black/black lobes contrast and multiply dislocations when the defect clusters grow to a critical diameter of 1.0-1.5 mu m. Our analysis with transmission electron microscopy and numerical calculations lead to a conclusion that these defect clusters are charged oxygen platelets formed by the selective displacements of oxygen sublattice.