Abstract: Anti-wear and friction reducing properties of four kinds of lubricant oils, N68, N68MD, Mobil SAE40 and N68ME, were examined with an X-P type pin-on-disk apparatus and a four-ball machine. Relationships between friction coefficient and time were studied and lubrication mechanisms of additives were investigated. Morphology and surface element distribution of the worn surfaces were analyzed by scanning electron microscopy (SEM) and electron dispersion spectrums analysis (EDS) respectively. It is found that friction coefficient of samples lubricated with N68MD decreased with time. Tribo-pairs lubricated with N68MD exhibited lower friction coefficient and lower wear rate. This was probably because N68MD contains elements of SP and Mo which could form a thin sediment layer on the worn surface of steel ball. Tribo-pairs lubricated with N68ME exhibited good extreme-pressure performance and very low friction coefficient, and with good self-repairing ability. This was probably caused from synergistic action of elements of SPMo, and nano-copper et al contained in the N68ME.