We investigated high temperature strength change of Mg2Si precipitate hardening type aluminum alloy with TEM observation as light material for a fuel cask. We tested strength changes during both an isothermal holding test at 230°C and a continuous cooling test from 226°C to 191°C up to 10.0kh. In the isothermal holding test Mg2Si was rod like precipitate parallel to ‹100›Al and showed like Ostwald ripening. In the continuous cooling test the kinetics of Mg2Si growing can also be explained by an extensive Ostwald ripening theory. In the isothermal holding test high temperature strength gradually decreased. But in the continuous cooling test the strength turned to increase after 3.0kh. From Ashby's precipitate hardening theory we can consider that is because that the decrease of Mg2Si precipitate hardening effect is smaller than the increase of matrix strengthening with decrease of temperature after 3.0kh.
