It has been attracted increasing interest and attention of researchers and engineers that the trace rare lanthanum and cerium addition increased the proportion of the equiaxed grains and led to grain refinement in solidified microstructure of steels. Generally, it was considered that such grain refinement was attributed to the heterogeneous nucleation of the high-melting-point rare earth inclusions when the rare earth elements exist in the form of inclusions in steel. The inconsistent previously reported results drives the in-depth study of the grain refining efficiency of these inclusions for steel castings. In the present work, the edge-to-edge matching model was used to calculate the atomic matching between the RE inclusions, namely Ce2O3 and La2O3, and the primary phase (both δ-Fe and γ-Fe), in order to predict the possibility and the potency of RE2O3 acting as the heterogeneous nucleation sites in liquid steel and also provide fresh insight into the understanding of the grain refinement mechanism of rare earth in the solidification of steels from the point of view of crystallography. The results show that both Ce2O3 and La2O3 are potent nucleant for heterogeneous nucleation of both δ-Fe and γ-Fe primary phases during solidification of steels. But, the grain refining potency of Ce2O3 and La2O3 for δ-Fe is higher than that for γ-Fe. Moreover, for γ-Fe, the La2O3 is more efficient than Ce2O3.