Abstract: The motion behavior of Al₂O₃ nano-particles with a size of 20 to 40 nm in high temperature liquid iron was studied. The results showed that after the nano-particles were kept on for 60 min in liquid iron, their size was still 20 to 60 nm; moreover, they did not become micro-particles and disperse better in liquid iron. The motion behavior of the nano-particles in liquid iron was described by the Brown theory. It is indicated that the phenomena of collision, reuniting and agglomeration of the nano-particles do not occur in liquid iron, and the motion behavior of the nano-particles described by the Brown theory is consistent with experiment results. The motion behavior of Al₂O₃ nano-particles is in contrast with usual Al₂O₃ non-metallic inclusions in liquid steel.