As an active metal, chromium is easy to form oxides film on the surface, which acts as an obstacle to the nitrogen diffusion in plasma nitriding. Rare earth (RE) was introduced into the conventional plasma nitriding (CPN) to improve the nitriding behavior. Chromium coatings were treated by CPN and rare earth plasma nitriding (REPN) at 620°C for 5 and 10 hours respectively. Comparative study showed that the addition of RE prompted greatly the formation of higher nitrogen containing phase CrN, rather than Cr2N. It created a crack-free, much denser and thicker nitrides layer, which contributed to the increase of hardness. The dynamic ultra micro hardness of the samples treated by REPN was nearly 1600GPa, which was much higher than the original and the CPN treated samples. The conclusion could be drawn that compared to CPN, REPN are more efficient for chromium coating nitriding.