Abstract: This study on the evolution behavior of the the Non-metallic inclusion in the steel was conducted systematically by the means of the oxygen and nitrogen analysis,microscopic inclusion statistics and SEMEDS energy spectrum.The CaS-CaO-Al₂O₃ tenary phase diagram in 1 600 ℃ was calculated and plotted by Factsage software.The evolution behavior of inclusions was analysed in the CaS-CaO-Al₂O₃ tenary phase diagram.It can be found that,under the current technological conditions,The total oxygen content and number of non-metallic inclusion in the molten steel has been decreased by the LF calcium wire-feeding treatment process.From the end of metal tapping to the tundish of caster,the total oxygen content has been decreased by 27×10^-6.The number of non-metallic inclusion has been decreased by 54.5%.The content of the total oxygen and nitrogen and the number of the non-metallic inclusion in the steady billet is low.The main components of microscopic inclusion is Al₂O₃ before the calcium treatment in the LF.There are a great number of the globular CaO-Al₂O₃ inclusion observed in the sample after the calcium treatment,in the LF and in the Tundish and slab.This research results indicates that calcium treatment achieves good results and the objective of the modifing alumina inclusion has been achieved.The thermodynamic calculation results show that the evolution behavior of inclusion is as follows:Al₂O₃→Al₂O₃+CaO·6 Al₂O₃+CaS→Al₂O₃+CaO· 2Al₂O₃+CaS（higher CaS and less CaO）→Al₂O₃+CaO· 2Al₂O₃+CaS（less CaS,higher CaO）.