Zr-ion irradiation effects on superplastic behaviors of 3mol% yttria containing tetragonal zirconia polycrystals (3Y-TZP) were investigated. Microstructure of 3Y-TZP specimens, irradiated with Zr ions, was observed by a scanning electron microscope (SEM) after superplastic bending test at 1623K. As a result, it was found that the Zr-ion irradiation probably induced microstructural changes during the superplastic bending deformation. Monte Carlo simulation showed that the Zr-ion irradiation induced vacancies and excess Zr ions within a range of 10 mum in depth. It was roughly comparable the range in which the microstructural changes were observed. The vacancies and excess Zr ions would cause the microstructural changes and probably restrained the diffusion-controlled grain boundary sliding in the superplastic deformation process. Hence it should result in an increase in flow stress and activation energy for the superplastic deformation as obtained in our previous study.