Abstract: The friction and wear tests of the plasma-nitrided layer on a 2Cr13 steel under perfluoropolyether (PFPE) type grease-lubricated condition in vacuum were conducted on a pin-on-disk tribometer. The morphologies of the worn surfaces were observed by using scanning electron microscopy. The chemical states of typical elements on the worn surfaces were examined by means of X-ray photoelectron spectroscopy. The chemical compositions of grease before and after friction tests were analyzed by using fourier transform infrared spectroscopy. Experimental results show that under grease-lubricated condition, a transition of wear mechanisms from a mild wear to a mixed mild wear with increasing the normal load and the sliding velocity was observed. Tribochemical reactions between the nitrided layer and PFPE grease during sliding in vacuum gave birth to the product of FeF₃. FeF₃ can be a catalyst to promote the degradation of PFPE grease to release acid fluorides. Increasing the normal load and the sliding velocity would accelerate the reactions between the nitrided layer and grease, and further promote the degradation of grease.