Nanostructured Al2O3 powders have been synthesized by combustion of aluminum powder in a microwave oxygen plasma, and characterized by X-ray diffraction and electron microscopy. The main phase is gamma-Al2O3, with a small amount of delta-Al2O3. The particles are truncated octahedral in shape, with mean particle sizes of 21-24 run. The effect of reaction chamber pressure on the phase composition and the particle size was studied. The gamma-alumina content increases and the mean particle size decreases with decreasing pressure. No alpha-Al2O3 appears in the final particles. Electron microscopy studies rind that a particle may contain more than one phase.