A Mg-Li-Al-Zn alloy was friction stir processed (FSP) under water, and the microstructures and superplastic behavior in the FSP alloy were investigated. The FSP Mg-Li-Al-Zn alloy consisted of a mixed microstructure with fine, equiaxed, and recrystallized alpha (hcp) and beta (bcc) grains surrounded by high-angle grain boundaries, and the average grain size of the alpha and beta grains was similar to 1.6 and similar to 6.8 mu m, respectively. The fine alpha grains played a critical role in providing thermal stability for the beta grains. The FSP Mg-Li-Al-Zn alloy exhibited low-temperature superplasticity with a ductility of 330 % at 100 A degrees C and high strain rate superplasticity with ductility of a parts per thousand yen400 % at 225-300 A degrees C. Microstructural examination and superplastic data analysis revealed that the dominant deformation mechanism for the FSPed Mg-Li-Al-Zn alloy is grain boundary sliding, which is controlled by the grain boundary diffusion in the beta phase.