Effects of the Retrogression and Reaging (RRA) treatments on the strength and the stress corrosion crack propagation in a high purity Al-Zn-Mg alloy were studied by measuring the hardness change and the crack growth rate as a function of the stress intensity factor using C-T specimens TEM was used to examine microstructural changes in both matrix and grain boundaries. The results showed that the RRA treatment resulted in a significant decrease of crack velocity without sacrificing T6 strength, similarly to the case of the commercial 7075 Al alloy. The RRA treated specimen showed a high overall concentration of precipitates in matrix including fine dispersion of η`. The improved SCC resistance of the RRA treated specimen is believed to closely related to increase in the size of grain boundary precipitates during the retrogression treatment.