Microstructure evolution and superplastic behaviors of ZK40 magnesium alloy were investigated in the temperature range of 473 similar to 623 K. 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 mu m. 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 low as 0.8 mu m; this alloy exhibited low temperature superplasticity at 473 similar to 523 K, elongations at an initial strain rate of I X 10(-3) S(-1) were 260 % at 473 K and 612 % at 523 K, respectively. The incompatibility between fine and coarse grains was thought to be unfavorable to the improvement of superplasticity. Low temperature superplasticity of the as-ECAP ZK40 alloy can be attributed to the relatively finer grain size and the homogeneity of microstructure.