Mullite-spinel-zirconia composites were prepared by reaction sintering of calcined alumina and magnesia, and zircon powders. The influence of calcining temperature on densification processes and on mechaical properties of subsequently sintered compacts was investigated. The mullite was formed by the reaction of $alpha$-Al2O3 and amorphous SiO2 at firing temperatures over 141$0^{circ}C$. The mullitization proceeded more rapidly in the specimen calcined at 110$0^{circ}C$ than at either 120$0^{circ}C$ or 130$0^{circ}C$. Microstructures before and after the mullitization (or mullite dissociation) showed different morphologies, and their effects on mechanical properties were significant. The flexural strength and fracture toughness of the specimen calcined at 130$0^{circ}C$ and subsequently fired at 145$0^{circ}C$ were 316 MPa and 4.2Mpa.m1/2, respectively.