The influence of Zn and/or Ag additions on the microstructure and properties of Al-Mg based alloys has been investigated. In the as cast condition, β-Al ₃Mg ₂ was dominated in Al-5 wt% Mg alloy with the addition of 0.66 wt% Zn. However, a further addition of Zn up to 2.55 wt% promotes the formation of T-Mg ₃₂(Al,Zn) ₄₉ phase. With the addition of Ag, no new intermetallic phase was observed. However, Ag partitions into the T-Mg ₃₂(Al,Zn) ₄₉ phase and thereby the T-Mg ₃₂(Al,Zn) ₄₉ phase transforms to Ag-containing T phase (T-Mg ₃₂(Al,Zn,Ag) ₄₉ phase) during solidification. After ageing treatment, the addition of Zn can promote the precipitation kinetics and thereby improve the strength of Al-Mg based alloys. Similar to the addition of Zn, the addition of Ag also promotes the precipitation kinetics and improve the strength of Al-Mg based alloys. This investigation can provide guidance for the alloy design of Al-Mg based alloys.