In the present study the effect of mechanical activation on combustion synthesis of Al2O3-B4C composite materials in a microwave using metal powder Al, graphite and boric acid is studied. The results showed that mechanical activation have an important role to promote exothermic reaction and Al2O3-B4C composite synthesis via combustion method. The mechanical activation decrease reaction temperature to lower than 700°C and increase reaction intensity without any effect on combustion reaction mechanism, so that, the sample synthesis was completely carried out in microwave at 12 second and 20 hours pre-mechanical activation. The results also support the possibility of boric acid using as raw material to combustion synthesis Al2O3-B4C composites.