Based on the two-step nucleation mechanism, a multi-step thermodynamic model for alumina inclusion formation during aluminum deoxidation process was proposed in Fe–O–Al melt. Thermodynamic properties of metastable intermediates including(Al2O3)nclusters for prenucleation and a-Al2O3 nanoparticle for growth process were calculated using density functional theory. Furthermore, Gibbs free energy change of forming the intermediate by reaction between the dissolved aluminum(Al) and oxygen(O) in the melt was calculated. The results indicated that the thermodynamics of(Al2O3)nat steelmaking temperature are dependent on their structures, while that of a-Al2O3 nanoparticle are dependent on their size. The nuclei of a-Al2O3 which was originated from(Al2O3)naggregated under a high supersaturation ratio of Al and O(Rs) in the melt. There existing excess oxygen because of the low Rs, but the secondary inclusions will be formed during the cooling process due to the excess oxygen. The nuclei lager than 20 nm can grow up spontaneously and instantaneously into primary inclusions because of thermodynamic drive. It is difficult to control the size of a-Al2O3 to be less than 20 nm, in the aluminum deoxidation process of the current conditions of steelmaking. Based on the two-step nucleation mechanism, a multi-step thermodynamic model for alumina inclusion formation during aluminum deoxidation process was proposed in Fe-O-Al melt. Thermodynamic properties of metastable intermediates including (Al2O3) nclusters for prenucleation and a-Al2O3 nanoparticle for growth process were calculated using density functional theory. Further, Gibbs free energy change of forming the intermediate by reaction between the dissolved aluminum (Al) and oxygen (O) in the melt was calculated. The results indicated that the thermodynamics of (Al2O3) nat steelmaking temperature are dependent on their structures, while that of a-Al2O3 nanoparticle are dependent on their size. The nuclei of a-Al2O3 which was derived from (Al2O3) naggregated under a high supersaturation ratio of Al and O (Rs) in the there exists excess oxygen because of the low Rs, but the secondary inclusions will be formed during the cooling process due to the excess oxygen. The nuclei lage r than 20 nm can grow up spontaneously and instantaneously into primary inclusions because of thermodynamic drive. It is difficult to control the size of a-Al2O3 to be less than 20 nm, in the aluminum deoxidation process of the current conditions of steelmaking.