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A 2Cr13 steel was modified by plasma nitriding. The microstructure, phase composition and microhardness of the nitrided samples were examined. The friction and wear behaviour of the nitrided and unnitrided samples sliding against self-mating under PFPE grease-lubricated conditions in vacuum was investigated on a pin-on-disk type tribometer, with the interactions among the nitrided layer and grease to be focused on. The morphologies of the worn surfaces were observed using a scanning electron microscope. The chemical states of several typical elements on the worn surfaces were examined by means of X-ray photoelectron spectroscopy. The chemical compositions of grease samples taken from the worn surfaces were analyzed by Fourier transform infrared spectroscopy (FTIR). Results show that the nitrided steel has better friction-reducing and anti-wear abilities than the unnitrided one under PFPE grease-lubricated conditions. In the former case, the wear of nitrided steel shows a transition from mild abrasive wear to fatigue one with increasing normal load. In the latter case, wear mechanism of unnitrided steel under all load applied is a combination of the severe abrasion and fatigue.