Abstract: The thermodynamic analysis and experiments of inclusions formation in a Ti-Al-O system in molten steel at 1 873 K were investigated. The results show that in the Ti-Al-O system the equilibrium between Al and O can be obtained instantly and the content of[O] is determined by the amount of Al. The formation of composite inclusions in the Ti-Al-O system depends on a_Ti/a_Al values. According to thermodynamic data, the stable section in the phase diagram of composite inclusions (Al₂O₃-Ti₃O₅-Ti₂O₃) in the Ti-Al-O system was calculated. The composition of inclusions changes from Al₂O₃ to Ti₃O₅ as the a_Ti/a_Al value increases. When the molten steel contains high amount of[O], it is very easy to form Ti₃O₅ inclusions. The experimental results agree well with thermodynamic data.