Abstract: Electron paramagnetic resonance(EPR) and cross-sectional transmission electron microscopy were used to study defect structure and radiated damage mechanism. Single crystal samples of cubic zirconia stabilized by 9.5 mol.% Y2O3 (YSZ) were implanted with 200 keV Xe ions and 400 keV Cs ions up to a dose of 5×1016 cm-2, respectively. EPR spectra show the trigonal signal with g‖=1.989 and g⊥=1.869, which exhibites axial symmetry with ［111］ direction as symmetry axis composed of sixfold-coordinated Zr3+ sites. Peak-to-peak intensity (per unit volume) of Cs-ion irradiated YSZ is as about 150 time as that of Xe-ion irradiated YSZ, which indicates that the concentration of sixfold-coordinated Zr3+ defects produced by Cs-ion irradiation is far more than that of Xe-ion irradiated. The cross-sect