The evolution of microstructural parameters (mean size and volume fracti on) of precipitates in three types of Al_Zn_Mg_Cu alloys during ageing has been studied by synchrotron_radiation small angle x_ray scattering (SAXS). The influe nce of Zn content on the precipitation has been analyzed. The results show that the precipitates are in nanometer scale for the three types alloys (A, B, C) . Th e maxima of the precipitate volume fractions forA, Band Cincrease wi th Zn content and are about 0.023—0.028, 0.052—0.054 and 0.04 respectively The evolution of volume fra ction at a certain temperature is consistent with the kinetic process of precipi tate nucleation, growth and Ostwald ripening.