Abstract: The hardness and chemical composition of plasma nitrided AISI 52100 steel surface were analyzed by means of X-ray diffraction (XRD) and hardness tester. Friction and wear behaviors of plasma nitrided AISI 52100 steel were examined on a four-ball friction and wear tester under lubrication of poly-α-olefin using ZDDP as an additive. Anti-wear and friction-reduction mechanisms were investigated using scanning electron microscopy (SEM) and X-ray photoelectron spectrometer (XPS). The results show that plasma nitriding can significantly improve the surface hardness of steel. The nitrided steel exhibited lower friction coefficient and better anti-wear properties compared with that of the untreated steel. It was attributed to that the amount of orthophosphates on the nitrided AISI 52100 steel surface was higher than the amount of pyrophosphates on the AISI 52100 steel surface. The best tribological property can be obtained using 1.5% ZDDP.