To verify the formation behaviors and mechanisms of intra-granular acicular ferrite( IAF) grains nucleated by Mg-Al-O in low carbon steel,the steels containing different Mg contents were refined in a vacuum induction furnace. The effect of Mg addition on the formation of IAF structure in Al-killed low carbon steel was investigated by optical microscope( OM) and scanning electron microscope with energy dispersive X-ray spectroscope( SEM-EDX). It reveals that the IAFs are only detected in Mg-added steels,and the volume fraction of IAF increases with the Mg concentration from 8 × 10~(-6) to 26 × 10~(-6). It shows that not only the MgO-Al_2O_3-MnS and MgO-Al_2O_3-P_2O_5 particles are the effective nucleation sites for IAF,but also the pure MgO·Al_2O_3 phase can promote the ferrite nucleation. A Mn-depletion zone( MDZ) is characterized adjacent to the MgO-Al_2O_3-MnS,which is believed to be one of the possible mechanisms to explain the IAF nucleation. The MDZ around the MgO-Al_2O_3-MnS inclusion would be induced by the Mn S precipitation on the inclusion. It seems that the ability of Mg-containing inclusions to induce the nucleation of ferrite might be attributed to a new mechanism,i. e.,the Prich zone formed on a few Mg-Al-O inclusions might be another factor for promoting the IAF formation. To verify the formation behaviors and mechanisms of intra-granular acicular ferrite (IAF) grains nucleated by Mg-Al-O in low carbon steel, the steels containing different Mg contents were refined in a vacuum induction furnace. The effect of Mg addition on the formation of IAF structure in Al-killed low carbon steel was investigated by optical microscope (OM) and scanning electron microscope with energy dispersive X-ray spectroscope (SEM-EDX). It reveals that the IAFs are only detected in Mg-added steels, and the volume fraction of IAF increases with the Mg concentration from 8 × 10 -6 to 26 × 10 -6. It shows that not only the MgO-Al 2 O 3 -MnS and MgO-Al 2 O 3 -P 2 O 5 particles are the effective nucleation sites for IAF, but also the pure MgO · Al 2 O 3 phase can promote the ferrite nucleation. A Mn-depletion zone (MDZ) is characterized adjacent to the MgO-Al 2 O 3 -MnS, which is believed to be one of the possible mechanisms to explain the IAF nucleation. The MDZ around the MgO-Al_2O_3-MnS i It seems that the ability of Mg-containing inclusions to induce the nucleation of ferrite might be attributed to a new mechanism, ie, the Prich zone formed on a few Mg-Al- O inclusions might be another factor for promoting the IAF formation.