The quenched M50NiL steel was plasma nitrided at 560℃ with and without rare earth (RE) addition. The effect of pure RE lanthan m (La) on the wear behavior of M50NiL steel plasma nitrided was investigated using pin-on-disk tribometer. The treated surfaces were characterized using SEM equipped with EDS, XRD and microhardness tester. The results show that the presence of La obviously changes the surface morphologies and phase structures of the nitrided layers. The Fe4N phase appeared on the plasma nitrided surface is replaced by the LaFeO3 phase after introducing La into nitriding atmosphere. The addition of La could obviously decrease the nitrogen content of the treated surface without reducing the surface hardness dramatically, which indicates that the LaFeO3 phase has a relatively high hardness. The wear rates of plasma nitrided specimens are apparently lower than that of untreated one. The involvement of La could further reduce the wear rates of plasma nitrided specimens. The specimen plasma nitrided for 8h with La addition demonstrates the best wear resistance with the wear rate of 0.93×10-5mm3N-1m-1. The transition of wear mode between adhesion and abrasion are mainly due to the differences in the phase structures and their proportion in the nitrided layers.