It is known that Al-Mg-Ge alloy shows the similar precipitation sequence to Al-Mg-Si alloy, and its equilibrium phase is β-Mg 2Ge according to its phase diagram. In this study, the precipitation sequence of Al-Mg-Ge alloys containing different contents of Mg 2Ge has been investigated by hardness test, TEM and HRTEM observation to understand the effect of Mg 2Ge contents on age-hardening behavior of the alloys. The hardness of as-quenched and peak-aged samples have been improved by increasing Mg 2Ge contents. The precipitates in the peak-aged samples have been classified into some metastable phases, such as the β'-phase and parallelogram-type precipitate by HRTEM observation. The relative frequency of these precipitates in the is has been changed with Mg 2Ge contents.