The biaxial stretching behavior of superplastic Y -TZP (yttria -tetragonal zirconia polycrystals) ceramics and Y -TZP/Al2O3 composites in air atmosphere was investigated. Stretching deformation can be made within a limited range. Larger grain size of Y -TZP led to higher applied load needed to obtain the same amount of deformation. However, if some monoclinic phase was present in the materials, which was caused by the over -large grain sizes, defects would occur and thus make deformation easier. Microstructure study of the deformed specimen showed the occurrence of crack -like defects on the outside surface of the stretched disk specimen. Limited grain growth of single phase Y -TZP was obtained while Y -TZP grains in Y -TZP/Al2O3 composites grew extensively after stretching.