Al₂O₃/aluminate composites were fabricated by a solid reaction in the La₂O₃-Al₂O₃ system. The formed phase showed elongated morphology that could be controlled by varying the hot-pressing temperature and time. Dense Al₂O₃/LaAl₁₁O₁₈ composites were fabricated by hot-pressing above 1500°C. When the hot-pressing temperature was 1400°C, Al₂O₃/LaAlO₃ composites were formed from starting composition. The grain size of monolithic alumina ceramics increased with increasing hot-pressing temperature, and marked grain growth occurred above 1600°C. However, the elongated LaAl₁₁O₁₈ phase suppressed the growth of alumina grains in the Al₂O₃/LaAl₁₁O₁₈ composites. Small LaAl₁₁O₁₈ grains existed in the alumina grains as an intragranular phase, while needlelike grains with a large aspect ratio existed at Al₂O₃ grain boundaries. The amount of in-situ formed LaAl₁₁O₁₈ phase increased with increasing amount of La₂O₃, and the aspect ratio of LaAl₁₁O₁₈ grains increased with increasing hot-pressing time.