The microstructure of a carbonitrided layer and its wear resistance were investigated by adding Rare Earth (RE) into 40CrMoSi bearing steel. The results indicated that while the addition of RE did not increase the depth of the carbonitrided layer, it did lead to an increase in both the size and content of carbides on the surface of the carbonitrided layer, thereby significantly improving its wear resistance. The underlying mechanism is as follows: RE compounds increase carburizing efficiency, which in turn increases the surface carbon concentration. Moreover, solid-solution RE atoms delay carbon diffusion into the center of the carbonitrided layer. Consequently, the carbon content is enriched on the surface of the carbonitrided layer, resulting in an increase in carbide size and quantity. These carbides, which are distributed on the surface of the carbonitrided layer, effectively increase the resistance of the martensitic matrix to plastic deformation and reduce minor cutting phenomena, thereby significantly improving the wear resistance of RE-modified 40CrMoSi bearing steel.