Al2O3 ceramics with magnesium aluminum spinel dispersion particulates were produced by a precipitation treatment of Al2O3-matrix solid solution, (Al1-2x,Ti-x,Mg-x)(2)O-3. We successfully constructed the precipitation process for synthesizing the nanocomposite, using the solubility dependence of titanium and magnesium in Al2O3 on the valence of titanium. Changing of the valence of titanium from 4+ to 3+ was accomplished by controlling the heating atmosphere, and, thereby, magnesium precipitation was promoted. The precipitation behavior was characterized using X-ray diffractometry, and the microstructure was observed using transmission electron microscopy, We confirmed that magnesium aluminum spinel nanosized particulate were precipitated in the Al2O3 grain.