Effective diffusivity for superplastic flow was investigated using a relatively coarse-grained (17 and 30 mum) Mg-Al-Zn alloy. Tensile tests revealed that the strain rate was inversely proportional to the square of the grain size and to the second power of stress. The activation energy was close to that for grain boundary diffusion at low temperatures, and was close to that for lattice diffusion at high temperatures. From the analysis of stress exponent, grain size exponent and activation energy, it was suggested that the dominant diffusion process was influenced by temperature and grain size. It was demonstrated that the notion of effective diffusivity explained the dominant diffusion process.