Evolution of flow stress and microstructures of an Al-Zn-Mg series alloy during deformation at elevated temperatures and large ranges of strain rates have been investigated by isothermal compression testing on Gleeble 1500 thermomechanical simulator. The results showed that the flow stress exhibited a characteristic of peak stress followed by smoothing out in the imposed strain rate and temperature ranges. The hot deformation of the alloy is a thermally activated process, which is controlled by rate-controlling mechanisms of dislocation generation and dislocation annihilation. The flow stress decreases with the increase of true strain at higher temperature and lower strain rate, indicating that dynamic softening dominates instead of strain hardening during hot deformation of the alloy. Microstructure observation revealed that typical subgrain cluster structures obviously developed in the original elongated grains of the alloy during hot deformation. The subgrain size of the alloy increased with deformation temperature and downturn of strain rate, where the Zener-Hollomon parameter increased. Fine equiaxed recrystallized grains with large-angle grain boundaries developed significantly in the alloy during hot compression at low Z values, implying activation of dynamic recrystallization.