Various grain-sized AZ80Mg alloys from 0.53μm to 16 μm were fabricated by multi-directional forging under decreasing temperature conditions. They were deformed in compression at some true strain rates from 5.0 × 10-4 s-1 to 5.0 × 10-1 s-1 and at temperatures between 423 K and 673 K. The coarser-grained samples showed large anisotropy of high-temperature deformation behavior and weak dependencies on temperature and strain rate. On the other hand, those of the finer-grained ones exhibited opposite behaviors. Only simple grain coarsening occurred in the finer-grain sized samples during hot deformation, while twinning and kinking as well as dynamic recrystallization took place in the coarser grain-sized ones. These grain-sized dependent deformation behaviors are precisely discussed from the differences in evolution of deformation-induced microstructures, thermal stability and grain-boundary sliding.