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 γ-alumina. At 10 Torr of oxygen, on the other hand, γ-alumina powders are produced with a small amount of aluminum powders. Annealing characteristics in nitrogen have also been studied on the transition from γ- to α-alumina.