Abstract: Based on the thermodynamic theory, the adiabatic temperature of NiO/Al aluminothermic system was calculated by computer numeral simulations. The results show that the adiabatic temperature is equal to the boiling point of Ni element (3156 K) when the preheat temperature is below 2790 K. That is to say, to preheat reactants only increases the gasification rate of Ni element. The results also indicate that the just concentration of Al₂O₃ diluting agent has little effect on the adiabatic temperature of NiO/Al aluminothermic system, which corresponds to phase transformation temperatures of productions.