The influence of nano-size WS2 powders on the tribological behavior of ta-C coatings by the filtered cathodic vacuum arc (FCVA) method under boundary lubrication conditions has been investigated. In order to characterize and understand tribological behaviors of nano-size WS2 powders added to the synthetic oil (poly-alpha-olefin 6), lubricants with different mixture ratios, ranging from 2 to 8 wt%, have been prepared. ta-C coatings fabricated by FCVA method showed that the G-peak in the obtained Raman spectrum was shifted from 1520 to 1586 cm(-1), indicating the sp(3) content increased for samples with the thickness of 156 nm. The average friction coefficient decreased proportionally as the nano-size WS2 compositions increased up to 4 wt% in PAO6. After the friction test, structures and particle sizes of WS2 phases were also precisely characterized by using XRD and SEM.