Abstract: In the current study, Al–Ti–O inclusions after Ti-alloyed in an ultra-low carbon IF steel were analyzed. It was found that Al–Ti–O inclusions were classified into seven types based on their morphologies, including four types with an Al₂O₃ outer layer and the other three without the Al₂O₃ outer layer. Approximately 78.0% of Al–Ti–O inclusions had an Al₂O₃ outer layer. There was little separated TiO_x inclusion detected in the steel. Without the consideration of the Al₂O₃ layer of Al–Ti–O complex inclusions, the core of Al–Ti–O complex inclusions was generally similar to that without the Al₂O₃ outer layer. Compared with the sample at 1 minute after the Ti addition, the number density of Al–Ti–O inclusions without an Al₂O₃ outer layer in the sample at 4 minutes after the Ti addition decreased by 0.21 mm^?2, while the number density of Al–Ti–O inclusions with an Al₂O₃ outer layer increased by 0.19 mm^?2. After the titanium alloying process, a large number of Al–Ti–O inclusions without the Al₂O₃ outer layer were transiently generated. Further, the Al₂O₃ outer layer was formed on the surface of inclusions, leading to the increase of the percentage of Al–Ti–O inclusions with the Al₂O₃ outer layer to 78.0%. Thermodynamic calculated results show that the evolution route of inclusions was solid Al₂O₃ → liquid Al–Ti–O → solid Ti₂O₃ with the increase of titanium content in the steel. The inclusion of Al₂O₃ was the only stable phase in the liquid steel in equilibrium, while the high concentration of titanium in the local steel during the titanium alloying process led to the formation of titanium-containing oxides. When the oxygen content in the steel was lower than 0.03%, inclusions were mainly solid Al₂O₃. Inclusions containing TiO_x were formed with oxygen content in the local steel exceeding 0.03% during the reoxidation process. The formation mechanism of Al–Ti–O inclusions was divided into two steps. After the titanium alloying process in the refining, when the local titanium content in the steel was higher than 0.42%, the Ti reacted with the molten steel to transiently form Al₂O₃–TiO_x and TiO_x. With the mixing of the titanium in the molten steel, the generated TiO_x-containing oxides were reduced by Al in the steel. Inclusions of Al₂O₃?TiO_x and TiO_x gradually transformed to Al₂O₃ on the surface.