Silicon carbide (SiC) and aluminium nitride (AlN) were found to form a solid solution at temperatures above 1800 degrees C. Based on this interesting result, the composite was fabricated by mechanical mixing of the SiC and AlN powders, and hot pressed under 40 MPa at 1950 degrees C in an argon atmosphere. The objective was to achieve full density and minimize solid solution formation. During the sintering process, the SiC-AlN system passed through three steps to form the solid solution at the end. First, the AlN particle is vaporized from its surface; next, the evaporated AlN is deposited on the surface of the SiC grains and the AlN particle, accompanied by a reduction in its size, and finally, partial SiC and AlN solid-solution formation on the boundary of the SiC grains. Because of the AlN deposition and solid-solution formation at the boundary of SiC grains, a barrier layer was present on the surface of SiC grains. This leads to the formation of fine grains. The toughening mechanism is thought to be by thermal residual stresses, due to the difference between the coefficients of thermal expansion of the matrix SiC and that of the dispersed AlN particles, and crack deflection around the SiC grains. Therefore, it is that which improves the mechanical properties of the SiC-AlN composite.