Abstract: This paper has used the equation of the superplastic mechanicsS (ij)=2/3K（ε e）{sup}(m-1)ε (ij)for the rectangular strip during the compressing of the superplastic state, and the theoretic formulae of the calculation of the deformation were obtained by the derivation, of The equilibrium approach and power approach. The unit average pressure was estimated to be approximately to the experimental value for the Zn-22%Al eutectoid alloy at the tempera Pure 250℃.