Nanosize powders of alumina have been synthesized by high‐density ablation plasma produced by the irradiation of an intense, pulsed, light‐ion beam on an aluminum target in oxygen. Diameters of the powders, which are observed to be spherical, are typically 5–25 nm. At 1 Torr of oxygen, nanosize powders of aluminum are synthesized with those of a small amount of &ggr;‐alumina. At 10 Torr of oxygen, on the other hand, &ggr;‐alumina powders are produced with a small amount of aluminum powders. Annealing characteristics in nitrogen have also been studied on the transition from &ggr;‐ to &agr;‐alumina. ©1995 American Institute of Physics.