A novel technique to synthesize mullite by the oxidation of Al-Si alloy powder was used for the manufacture of mullite via pressure molding. The mixed powder was consisted of powders of Al-Si alloy, Al2O3 and SiO2 and additives or powders of Al-Si alloy, Al2O3, SiO2 and ZrO2 and additives. Those mixed powders were milled in nonaqueous slurry in order to prevent the oxidation of Al-Si alloy. Then the green bodies were pressureless sintered in air at ≥1380 °C. The Oxidation caused an internal volume expansion, resulting in reducing shrinkage stresses during reaction- bonding of composites, which minimized the sensitivity to crack formation. Comparing with non zirconia reinforced mullite composites, the effect of the microstructure and mechanical properties of mullite composites were studied.