Abstract: Cu nanoparticles coated with dialkyldithiophosphate (Coded as Cu DDP) were synthesized in situ by redox method. The size and structure of the nanoparticles were characterized using TEM and ED. The tribological behaviors of Cu DDP as additive in liquid paraffin were evaluated and compared with that of zinc dialkyldithiophosphate (ZDDP) on a four ball machine. The results show that Cu nanoparticle possesses excellent load carrying capacity and good antiwear property, which are better than that of ZDDP. The lubricating mechanism of Cu nanoparticles was discussed based on the XPS and EPMA analytical results.