Superplastic behaviors were investigated for fine-grained yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) from a viewpoint of GeO2 or TiO2 doping. It was found that both dopants enhance the ductility in TZP. In particular, elongation to failure of 988% could be obtained in 3Y-TZP co-doped with 2mol%GeO2 and 2mol%TiO2. In addition, it was revealed that lower flow stress did not always give a larger elongation in this system. On the other hand, the strong segregation of dopants along grain boundaries was confirmed by high-resolution electron transmission microscopy study (HRTEM) with X-ray energy dispersive spectrometer (EDS). The unique phenomena observed in the relation between flow stress and ductility is closely related to the strong segregation of dopants along grain boundaries in this system. The grain size at fracture is determined by covalency because of the dopant segregation. It could be concluded that elongation to failure is closely related to a balance between grain size at fracture and grain growth rate during deformation.