The metastable β′-phase that forms in an alloy with composition Al-1.0 mass% Mg₂Si-0.5 mass% Ag, over aged at 523 K, has been investigated by transmission electron microscopy (TEM) in order to understand the effect of Ag-addition on the crystal structure of this phase. According to the results of analyses of selected area diffraction pattern, high resolution TEM (HRTEM), energy dispersive X-ray spectroscopy (EDS), the elemental maps by energy-filtered TEM (EFTEM) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), this precipitate consists of Mg, Si and Ag, and has a hexagonal unit cell which is similar to that of β′ phase in Al-Mg-Si alloys without Ag. However, the unit cell a-axis of β′ with Ag was 0.69 nm, which is slightly smaller than the corresponding dimension in Ag-free β′ (0.71 nm). The bond overlap population (BOP) was also calculated for the bonding between atoms in the small cluster in the β′-phase by the discrete-variational (DV)-Xa method, suggesting the bonding in β′ with Ag was higher than in the Ag-free β′.