Abstract: For Cr12 MoV steel produced by a domestic steel plant via EAF → LF → VD → die casting process,the amount of rare earth（La,Ce） added is 134×10^-6,and the samples is taken from liquid steel during VD process at different times.ASPEX automatic scanning electron microscope was used to analyze the composition,quantity and size of non-metallic inclusions in steel.It is found out that rare earth can improve the type and morphology of inclusions in steel,and transform more than 60% of irregular oxide and sulphide inclusions into spherical or ellipsoidal rare earth inclusions.The influence of rare earth on the amount and size of inclusions is related to the soft blowing time of the VD process.When the soft blowing time is controlled at 26 min～45 min,the number and size of inclusions in the steel are continuously reduced with the increase of soft blowing time.When the soft blowing time is prolonged to 55 min,the size of inclusions is still decreasing but the number is increasing,in particular to Al₂O₃ and REAlO₃ types.Considering the cost for inclusion modification and VD process for studied steel with 134×10^-6 rare earth（La,Ce） added,the optimal soft blowing time of the Cr12 MoV steel VD process should be controlled between 33～45 minutes.