The equilibrium phase and adiabatic temperature for combustion synthesis of Al2O3/B4C employing Al, B2O3 and C as starting materials is analyzed by both conventional and CALPHAD method. The adiabatic temperature calculed by CALPHAD method is significantly lower than that obtained by conventional method. The CALPHAD calculation also reveals that the equilibrium phases presented at the adiabatic temperature are different to the desired composites. The adiabatic temperature in this system can be lowered by introducing Al2O3 as diluents. The maximum amount of Al2O3 that can be added to the system while maintain a self-sustaining combusion mode is 1.3 mol.