The microstructures of precipitates in Al-Zn-Mg alloys in peak-aged condition have been studied using scanning transmission electron microscope. The same thermo-mechanical treatment was applied in all alloys. Investigation of peak-aged samples revealed that the most commonly found phases were eta' and eta (1) with their respective habit planes on {111}(Al) and {100}(Al). eta' phases under [110](Al) were analyzed and compared with eta' structure models. Furthermore, a close inspection of eta (1) phase as the second most found precipitate revealed that it incorporates an anti-phase resembling boundary, not observed in other orientation relationships that precipitates create with Al matrix, in addition, differences in matrix-precipitate interfaces between eta'/eta (2) and eta (1) phases were noticed. This paper addresses the first part to the analysis of eta' phase. Next part is extended to the analysis of the eta (1) phase.