In this paper, the morphology of CA₆ was adjusted by changing the particle sizes of alumina powders (D₅₀ = 7.26, 5.33 and 2.37 µm), and the effect of this on the thermo-mechanical properties of corundum based high temperature composites was investigated. The results indicated that when the alumina particle size was decreased from 7.26 to 2.37 µm, the morphology of calcium hexaluminate (CA₆) transformed from equiaxial to platy shape. The microstructure changes of CA₆ may be due to differences in CA₆ nuclei numbers and the contact area between alumina and calcium dialuminate. This transformation process can proceed without liquid. Because of the transformation of the CA₆ morphology, the hot modulus of rupture and residual cold modulus of rupture of corundum composites fired at 1600°C increased by 143 and 192%, respectively. The loop areas of in-situ elastic modulus also increased by 20%. The fracture ways of composites were transformed from intergranular to transgranular. Therefore, corundum-based composites with excellent properties including room temperature and high temperature properties can be obtained by adjusting the microstructure of CA₆.