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The mechanical and corrosion properties under various ageing treatment conditions were investigated in an Al-6.0Zn-2.3Mg-1.8Cu-0.1Zr (wt.%) alloy. The results showed that the retrogression and re-ageing (RRA) were capable of providing higher strength and improved corrosion resistance in comparison with the conventional T6 and T74 ageing. The optimised ageing process had been found to be 120°C/24 h + 180°C/60 min + 120°C/24 h for the experimental alloy. The results obtained from the high resolution transmission electron microscopy (HRTEM) interface analysis revealed that a semi-coherent stress field between the η′ precipitate and the Al matrix was critical in controlling the strength of the Al-Zn-Mg-Cu alloy heat-treated under different conditions. Furthermore, Transition Matrix calculation showed that the η′ phases had only two zone axes: [1 ̅ 21