Abstract: The Ti-Mg treatment was carried out in low carbon steel,and its effects on inclusion (type,size,distribution) and microstructure and the possible mechanism of inclusions induced nucleation had been studied by scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS),and optical microscopy (OM).It was found that after treatment,the number of inclusions per unit volume increased by 48%,and the inclusions whose size is less than 4 μm accounted for 77%.Microstructure observation con-firmed intra-granular acicular ferrites which is due to the nucleation effect induced by (Ti,Mg)O_x-MnS composite inclusions.Moreover,line scanning analysis indicated that Mn-depletion zone (MDZ) exist in steel matrix adjacent to the complex inclusion.This phenomenon can be explained by the absorption of Mn in (Ti,Mg)O_x,and this MDZ promotes the nucleation of intra-granular acicular ferrite.