Hot deformation of an Al-Zn-Mg alloy was physically modeled by isothermal compression, testing on Gleeble 1500 thermal mechanical simulator to investigate microstructure evolution of the alloy during deformation at elevated temperatures and large ranges of strain rates. Electron Back-Scattering Diffraction (EBSD) was conducted to study the evolution of the grain and subgrain misorientation. Evidence of optical microstructure observation and flow stress, as well as microtexture of the alloy, were analyzed. Material constants of the alloy were derived out to validate the relationship between subgrain size of the alloy and Zener-Hollomon parameter (Z parameter). The investigation indicated a decrease of flow stress of the Al-Zn-Mg alloy with increase of true strain, revealing the occurrence of strong dynamic restoration during hot deformation. Very fine equiaxed grains, which possess high-angle boundaries and different orientations to the matrix, were found to develop in the alloy compressed at higher temperatures and lower strain rates, implying the activation of geometric dynamic recrystallization in the alloy. The reciprocal of the subgrain size, which formed in the dynamically recovered zones, was found to have a good agreement in-a linear relationship with the logarithm of Z parameter, indicating thermal activation dominated in dynamic recovery process of the alloy.