In this paper, fast multiple rotation rolling ( FMRR) is applied to fabricate a nanostructured layer on the surface of steel 45. The FMRR samples are then Cr?Rare earth?boronized under low?temperature. The boride layer is characterized by using Scanning electron microscopy ( SEM ) and X?ray diffraction ( XRD ) . Experimental results indicate that the thickness of the boride layer is greatly increased by surface nanocrystallization. The boride layer with relatively continuous structure instead of the zigzag teeth structure is obtained, and the penetrating rate is enhanced by 2?5-3?7 times when the FMRR samples are Cr?Rare earth?boronized at the temperature of 570 ℃, 600 ℃ and 650 ℃ for 6 h. The boride layer fabricated on the FMRR sample consists of single phase Fe2 B. Severe plastic deformation with the grain size of approximately 100 nm in the top surface layer of steel 45 is observed, and the thickness of the plastic deformation layer is about 30μm. The microstructure in the top surface layer is characterized by Transmission electron microscopy ( TEM) . Grain boundaries are largely increased with high stacking fault energy after FMRR, leading to a significant enhancement of RE boron?chromizing speed.