A new finding is reported for the surface corrugation induced in the superplastic deformation and flow of 3 mol% yttria-partially stabilized tetragonal zirconia polycrystal and β-spodumene glass ceramic, the former including no secondary phase and the latter having a plenty of glassy phase at grain boundaries. A significant test specimen size effect on the resistance to superplastic deformation is also addressed. These findings are related to the microscopic processes and mechanisms of cooperative grain-boundary sliding.