Experiments were carried out using Al-6% Zn-1.8% Mg and Al-6% Zn-1.8%Mg-1.5%Cu alloys. Addition of Zr approximately 0.2% followed by ingot homogenizing at 450°C for 8 to 16 hr remarkably improves the strength and stress corrosion resistance of the alloys. Zr in the Al-Zn-Mg alloy precipitates in the form of metastable Al₃Zr during homogenizing. Precipitation of spherical Al₃Zr particles 100 to 500 Å in size and 0.1 to 0.5 μm in spacing inhibits growth of subgrains. The sensitivity of Zr-bearing alloys to quenching is slightly intensified in the above condition because of precipitation of stable MgZn₂ phase at the misfit surface between Al₃Zr and matrix during slow quenching.