Abstract: In order to promote hot metal desulphurization,Al is used to deoxidize during molten iron pretreatment. Because of the high carbon content of hot metal,Al and C exist in hot metal simultaneously. Determinant conditions of Al₄C₃ precipitation were investigated and the effect of Al₄C₃ on aluminum deoxidation was analyzed. The value of a_Al⁴·a_C3 product was calculated by using a thermodynamic method and then the equilibrium of Al-C-O was studied. According to the calculated value of a_Al⁴·a_C3 product,the Al-C equilibrium curve was plotted and the equilibrium was analyzed to determine the composition range in which Al₄C₃ can precipitate from hot metal.