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Nitriding can improve the surface hardness and wear resistance of metals, and rare earths can speed up this process. To study the mechanism of rare earths promoting salt bath nitriding, GX-8 steel was nitrided at different temperatures in two salt baths with and without rare earth Ce. First, structures and depths of the nitrided layers were investigated by microscopic observation, X-ray diffraction, GD-OES and microhardness tests. Then, the diffusion kinetics were calculated, and the atomic mechanisms of rare earth-assisted nitriding were studied by first principles. The kinetics calculations show that after adding Ce to the salt baths, the diffusion activation energy decreases, whereas the decomposition rate of active nitrogen atoms increases. The firstprinciples results indicate that Ce increases the energy and volume of the supercell of Fe-Cr alloy with N interstitial dissolution and reduces the stability of the supercell. These results demonstrate rare earths can accelerate nitriding at the atomic level.