The effect of cerium (Ce) on the solidification microstructure of Cr4Mo4V bearing steel was investigated via a combined experimental and theoretical method. With a trace amount (0.056 wt%) of Ce addition, the coarse columnar grains in as-cast microstructure transform into equiaxed ones, and the average diameter is reduced from 56 to 27 μm. The network-like and bulky primary MC and M2C carbides at the interdendritic regions become disconnected and refined, and their volume percentage decreases from 4.15 vol% to 2.1 vol%. Ce-inclusions acting as heterogeneous nucleation agents of prior-austenite grains and Ce atoms segregating at grain boundaries, both contribute to the refinement of grains. Thermodynamic calculations reveal that primary carbides are precipitated after γ-austenite forms near the end of the solidification process. The modification of primary carbides in size and amount is mainly attributed to the isolated remaining melt separated by refined γ-austenite grains in which the nucleation of carbides is promoted, while the growth is restrained owing to the less segregation of alloying elements. The grains and primary carbides of Cr4Mo4V bearing steel was refined with the proper addition of Ce. We find that Ce-inclusions acting as heterogeneous nucleation agents of prior-austenite grains and Ce atoms segregating at grain boundaries, both contributed to the refinement of grains. The modification of primary carbides in size and amount is mainly attributed to the isolated remaining melt separated by refined γ-austenite grains in which the nucleation of carbides is promoted, while the growth is restrained owing to the less segregation of alloying elements. Display omitted