A number of nanoscale precipitates with random orientations were produced by electron irradiation of polycrystalline Mg-doped alpha-Al2O3. The precipitation behavior and microstructural changes were observed using in situ high-resolution transmission electron microscopy. A ring pattern corresponding to a polycrystalline fcc structure was obtained from the region of these precipitates by selected-area diffraction, and they were identified as nonstoichiometric MgO.nAl(2)O(3). It is believed that Al2O3 with MgO(ss) and/or MgO at the grain boundaries is effectively stabilized by the electron irradiation process, forming the fundamental structure of MgO.nAl(2)O(3) The average size of the precipitates is about 10 nm.