Superplastic behavior in 0.2 mol% of cation doped fine-grained yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) was examined with a special interest in cation-segregation along grain boundaries. The flow stress of cation doped 3Y-TZP is correlated well with the ionic radius of doped cation independently of the solubility of each cation in TZP. HREM-nano probe EDS analysis indicates that the dopant effect on the superplasticity is caused mainly from the grain boundary segregation of doped cation.