Microstructure evolution and superplastic behaviors of ZK40 magnesium alloy were investigated in the temperature range of 473～623 K. Transmission electron microscopy (TEM) was used to study the microstructure changes, twinning occurred significantly after being processed by equal channel angular pressing (ECAP) for one pass through the die, the mean grain size was 5.6 μm. Finer grains can be obtained after further processing through ECAP, the average grain size of the alloy processed by ECAP for three passes was as Iow as 0.8 μm; this alloy exhibited Iow temperature superplasticity at 473～523 K, elongations obtained at the same initial strain rate of 1 × 10-3 s-1 were 260 % at 473 K and 612 % at 523 K,respectively. Corresponding values for the ZK40 alloy processed by ECAP for only one pass were 124 % at 473 K and 212 % at 523 K, respectively; poor superplastic behavior of this material was related to the long-range stresses associated with the non-equilibrium grain boundaries within the coarse grains. The incompatibility between fine and coarse grains was thought to be unfavorable to the improvement of superplasticity.