Hardness tests and transmission electron microscopy were used to investigate the strategy of tailoring the phase fraction of precipitates in an Al-Zn-Mg-Cu alloy strengthened by T'''' and η'''' phases. Different phase fractions of T'''' and η'''' phases are presented in samples subjected to either single or two stages of ageing treatments at 120 and 150 °C. For both types of ageing, the precipitation of η'''' phase is found to be promoted by ageing at lower temperature and its phase fraction increases with prolonging ageing time at 120 °C; whereas the phase fractions of T'''' and η'''' phases almost remain constant during ageing at 150 °C. Besides, the strain fields produced by T'''' and η'''' phases were analyzed by using the geometric phase analysis technique, and on a macroscale the contributions of T'''' and η'''' phases to precipitation strengthening have been quantitatively predicted by combining the size, phase fraction and number density of precipitates.