Superplatic behavior in GeO2 or TiO2 doped and Ge-Ti co-doped 3mol% yttria doped tetragonal zirconia polycrystals (TZP) are examined under a uniaxial tension in air at 1400degreesC and an initial strain rate of 1.3 X 10(-4)s(-1). Diffsivity on static grain growth behavior is enhanced by Ge-Ti co-doping. Dependence of the flow stress on the amount of co-doped Ge-Ti is close to that in Ge doped TZP, but much lower than that in Ti-doped TZP. The elongation to failure is highly improved by the co-doping in comparison with Ge or Ti doping. The largest nominal strain of 988% was obtained in 2mol%GeO2-2mol%TiO2 co-doped TZP.