Molecular dynamics simulations were carried out for model polycrystals of yttria-stabilized zirconia in order to investigate the microscopic mechanism of plastic deformation. The model polycrystals were composed of eight and sixteen grains with two- and three-dimensional grain arrangements, respectively. During the deformation, a neighbor-switching event similar to Ashby and Verrall's model was observed for both model structures. Further complex motions of the grains were observed in the case of the three-dimensional grain arrangement. By analyzing the time variation of the positions of oxide ions, mass transport during the deformation was found to take place mainly at the grain boundary regions.