Abstract: The microstructure and inclusions in rare earth treated C-Mn steel were studied,the size distribution of the nuclei inducing acicular ferrite nucleation was subjected to statistical analysis,and the formation and growth processes of rare earth inclusions were discussed through quenching the treated samples at different temperatures. It is found that after a small amount of rare earth is added into the C-Mn steel,the inclusion composition changes from Mn S + aluminosilicate to La₂O₂S + LaAlO₃+ MnS + aluminosilicates,the inclusion size decreases,and the microstructure converts from martensite + bainite to ferrite side plate + acicular ferrite+ massive ferrite. The size of effective inclusions inducing acicular ferrite nucleation is mainly concentrated in 1-4 μm in the rare earth treated C-Mn steel,they primarily form in liquid steel,but little in the cooling and solidification processes. The rare earth inclusions are easy to coalesce and grow up at molten temperature and in the cooling and solidification processes,and Mn S can precipitate around the residual rare earth inclusion particles.