The author's theory for plastic deformation was applied to superplastic alloys (Zn-Al eutectoid, Al-Cu, Pb-Sn, Sn-Bi, Mg-Al eutectics). The plastic deformation of the superplastic alloys could be described by two Maxwell models connected in parallel which represent two grain boundary flow units. The flow units are characterized by the two parameters

(j=l or 2, g signifies the grain boundary) the values of which were obtained by applying our flow equation [Eq. (5)] to experiment. We confirmed that our flow equation describes the superplasticity very well. The curve of strain rate sensitivity m (=

, where f and s are stress and strain rate, respectively, showed two peaks corresponding to flow unit gl and g2, the separation of the two peaks is determined by the difference between

. The condition of superplasticity is also determined by

, which satisfies

[Eq.(13)], where

is the s of the jth unit at the peak. The grain size dependence of

is described by

=alnx+b [Eq. (16)], where x is the grain size, and a and b are constants. The activation enthalpy for each flow unit,

was also determined from the temperature dependence of

which is proportional to the relaxation time of the j th unit. Since the superplasticity is determined by Eq. (13), and since

and

are related, we obtained the conclusion that superplasticity occurs in the system having small

values. The Aej values were equal to the activation enthalpies of grain boundary self-diffusion of the component atoms of the alloys, this accords with our proposed flow mechanism. The

value increases with grain size as expected from Eq. (16).