In order to ascertain superplastic mechanism and establish constitutive equation, strain rate sensitivity exponent (mvalue) is required to be calculated. Fine-grained AA7075 aluminum alloy sheets were fabricated by casting, rolling and annealing, and variation in the mvalues were investigated by high temperature tensile machine and image analyzer. Aiming at the superplastic deformation of materials with the equiaxed grain and the elongated grain microstructures, the relations between the mvalue and the strain were modeled. It is noted that the mvalue for the equiaxed grain microstructure decreases with increasing the strain during constant velocity tension, and the mvalue for the elongated grain microstructure increases with increasing the strain during superplasticity. Theoretical predictions were verified by the superplastic experimental evidence in AA7075 alloy and AA7475+0.7Zr alloy with fine equiaxed grains and in Mg-8.5%Li alloys with the elongated grains and AA7475 alloy with low angle grain boundaries. The calculated results agree well with the experimental ones, and it is shown that variation in microstructure leads to the variation in the mvalue.