In situ processing is an attractive and cost-effective approach to fabricate ceramic matrix composites with high performances. In this paper, the in situ synthesis of alumina-matrix oxide/oxide composites reinforced with Ba-β-Al 2 O 3 or Ba-β-Al 2 O 3 /ZrO 2 phases through reactive sintering of Al 2 O 3 /BaCO 3 or Al 2 O 3 /BaZrO 3 powder mixtures is briefly reviewed. The in situ synthesized composites exhibited a microstructural feature of elongated Ba-β-Al 2 O 3 and equiaxed ZrO 2 particles, which were distributed in Al 2 O 3 matrix homogeneously. The TEM observations suggest that the longitudinal interface between Ba-β-Al 2 O 3 phase and Al 2 O 3 matrix is parallel to the basal planes of the Ba-β-Al 2 O 3 phase. The formation of the elongated Ba-β-Al 2 O 3 particles is likely to be due to the preferred diffusion of Ba cations along (0001) basal planes. Furthermore, the diffusion of Ba cations dominates the in situ reaction processes. The presence of Ba-β-Al 2 O 3 and ZrO 2 phases resulted in crack deflection and crack bridging, thus improving the fracture toughness of the Al 2 O 3 matrix composites.