Grinding lanthanum oxide (La2O3) with Al2O3 was conducted to investigate their mechanochemical reactions to form lanthanum aluminate (LaAlO3) powder using a planetary ball mill. Grinding for 120 min allowed us to obtain single-phase LaAlO3, with a large surface area when transition alumina was used, whereas no formation of LaAlO3 was achieved when alpha-Al2O3 was used. The mechanochemical process can be applied to synthesize other rare-earth (RE) aluminates (REAlO3) from mixtures of a rare-earth oxide and transition alumina.