The effects of Zn/Mg ratios on microstructure and mechanical properties of Al-Zn-Mg-Cu alloys aged at 150℃ have been investigated by using tensile tests,optical metallography,scanning electron microscopy,transmission electron microscopy and atom probe tomography analyses.With increasing Zn/Mg ratios,the ageing process is significantly accelerated and the time to peak ageing is reduced.T\'phase predomi-nates in alloys of lower Zn/Mg ratios while η\'phase predominates in alloys with a Zn/Mg ratio over 2.86.Co-existence of T\'phase and η\'phase with a large number density is beneficial to the high strength of alloys.Such precipitates together with narrow precipitate free zones cause a brittle intergranular fracture.A strength model has been established to predict the co-strengthening effect of T\'phase and η\'phase in Al-Zn-Mg-Cu alloys,including the factors of the grain boundary,solid solution and precipitation.