Abstract: The effects of diluting agent Al₂O₃ on the combustion velocity and the products microstructure of NiO/Al aluminothermic system were studied. Combined with the adiabatic temperature calculation, the apparent activation energy of NiO/Al system were measured by Arrhenius diagram of ln(v/T_ad)-1/T_ad based on the combustion equation proposed by Merzhanov et al. The results show that in a certain range of Al₂O₃ mass fraction(0~5％, 25%~35％), the combustion velocity of NiO/Al aluminothermic system is considerable invariable. The further results show that the process of combustion is contributed to the mass-transferring of Al reactant of burning compact. When the temperature above the boiling point of Al(2740 K), the apparent activation energy of NiO/Al aluminothermic system is (64±14) kJ/mol. On the contrary, that is (189±15) kJ/mol.