Abstract A superplasticity mechanism based constitutive equation incorporating deformation-induced grain growth and deformation-accelerated grain boundary diffusion was established. The constitutive equation established was applied to the superplastic two-phase hypereutectic Mg–8.42Li alloy. The contribution of net static grain growth to entire grain growth accounts for 51.65% while the contribution of net strain grain growth to entire grain growth accounts for 48.35%. It is shown by calculation that the grain boundary diffusion in the superplastic deformation of fine grained microduplex Mg–8.42Li alloy at low strain rates is not accelerated obviously and deformation-accelerated grain boundary diffusion takes place at high strain rates. It is indicated by normalizing the data of calculation and experiment that the flow mechanism of this alloy at 573K at an initial strain rate of 5×10−4s−1 is modified Mukherjee's grain boundary sliding controlled by the motion of the dislocations in the grain boundaries by a climb-glide process and Gifkins's grain boundary sliding occurring by the motion of grain-boundary dislocations that pile up at triple point.