Abstract: The effect of 0.02wt%-0.06wt% added Ce/La composite rare earth（Ce 70wt%, La 30wt%） modification on the as-cast microstructure of W6Mo5Cr4V2 high speed steel was studied by OM and SEM. The results show that the dendrites of unmodified W6Mo5Cr4V2 steel are well grown, and there are a lot of carbides distributed along grain boundaries. After Ce/La composite rare earth modification, the secondary dendrite spacing in the as-cast microstructure of the steel decreases, the network carbide is interrupted, and the carbide is refined. Compared with those of unmodified the steel, the secondary dendrite spacing of the high speed steel with 0.04% Ce/La rare earth is reduced by 14.5%, the carbide network structure is basically broken, the area percentage is reduced by 33.7%, and the modification effect is the best.