Al 2 O 3 nanoparticles were prepared by polyethylene glycol octylphenyl ether(Triton X-100)/n-butyl alcohol/cyclohexane/water W/O reverse microemulsion. The proper calcination temperature was determined at 1 150 °C by thermal analysis of the precursor products. The structures and morphologies of Al 2 O 3 nanoparticles were characterized by X-ray diffraction, transmission electron microscopy and UV-Vis spectra. The influences of mole ratio of water to surfactant on the morphologies and the sizes of the Al 2 O 3 nanoparticles were studied. With the increase of surfactant content, the particles size becomes larger. The agglomeration of nanoparticles was solved successfully. And the formation mechanisms of Al 2 O 3 nanoparticles in the reverse microemulsion were also discussed.