The addition of light rare‐earth (RE) metal elements (La, Pr, and Nd) to Al thin films with about 2–7 at. % markedly decreases the grain size of the Al matrix more than 50% compared with those of pure Al. Such addition largely suppresses growth of thermal defects of hillocks and whiskers at high temperatures (350–450 °C). A large number of fine metallic compounds of Al11RE3and/or Al3RE (RE=La, Pr, and Nd) were segregated in an Al matrix, mostly at grain boundaries, after annealing at 350 °C. The resistivities of the films after annealing at the above temperatures show low values of less than 6 μΩ cm compared with those of current thin‐film‐transistor liquid‐crystal displays gate electrode materials (more than about 15 μΩ cm), without the salient formation of hillocks or whiskers on the surfaces.