Pure rare earth metal block La, Ce and Nd was put into the nitrided furnace respectively as a sputter source when typical nitride steel En40B was plasma nitrided. It was found that the incorporation of pure rare earth resulted in the changing of characteristic of glow discharge. Analysis of samples after treatment was performed using related instruments and facilities to examine the effects of treatment on microstructure, micro hardness and element distribution. The results showed that the incorporation of rare earth metals in the glow discharge influenced the diffusion of nitrogen in some degree. Thus the difference of distribution of elements, structure and hardness in the nitrided layer was formed compared with the conventional be plasma nitrided. These significantly affected the response of the investigated steel on plasma nitriding. The extent of influence to plasma nitriding varied with different rare earth.