The effectiveness of microalloying addition and two-step aging on the mechanical properties of the Al-Zn-Mg alloy has been investigated using TEM, tensile test and nanoindentation. By decreasing width of PFZ and size of grain boundary precipitates through the addition of (Ag+Sn) or two-step aging process, tensile properties of Al-Zn-Mg alloys are markedly improved. The elongation was quantitatively related to the three microstructural factors; i.e. the width of PFZ, size of grain boundary precipitates and the level of proof stress, to predict ductility of the alloys with known microstructural factors. The fracture mode change is reasonably in terms of the hardness difference between grain interiors and PFZ region by a noindentation technique.