To improve the property of Al-Mg alloy, increase of Mg concentration has been carefully studied even though serious problems associated with the precipitation of β(Al₃Mg₂), as like low thermal-stability in mechanical properties and low corrosion resistance. are generated with higher Mg concentration. In the present works, changes of β precipitate behavior, and mechanical properties by La and misch metal(Mm) addition in high Mg content Al-Mg alloy have been investigated and showed that precipitation of β phase is retarded and thermal stability of mechanical properties have improved by La, misch metal(Mm) additions.