A pin-to-disc wear testing machine was used in order to investigate the wear characteristics of a lubricant with nano-diamond (ND) particles.The applied loads were 30 N, 40 N, 50 N and 60 N and the sliding speed was 0.3 m/s.The wear tests were run with a sliding distance of 24 km.The friction coefficients and wear loss of S45C specimens were tested in a commercial oil (5W-40) with different ND concentrations ranging from 0 to 0.02 wt.%.The addition of ND particles to commercial oil was found to reduce the coefficient of friction for S45C steel.ND particles as a solid additive in oil lubricants were found to improve the tribological behavior of S45C carbon steel.For the S45C steel, the best concentration was found to be 0.0075 wt.% for both the anti-wear and friction reduction performance capabilities.