Most of lubricant additives used as friction modifier and anti-wear agents are mainly organic compounds containing sulphur and phosphorous. Their lubrication mechanism is based on a tribochemical reaction leading to tribofilm formation but also the formation of some harmful by-products. Inorganic nanoparticles (nanotubes, fullerenes, onions...), because of their unique morphology and very small size, could be envisaged for the replacement of such organic additives. The purpose of this work is to study and compare the tribological properties of different kinds of nanoparticles added and dispersed as additives to a lubricating base oil. Here, we are particularly interested in carbon nanotubes and graphite onions which were then tested and compared. Added to a poly-alpha-olefin (PAO) base oil, all nanoparticles tested show a reduction of both friction and wear of steel counterfaces. The detailed study of the concentration effect in PAO shows that lwt% of nanotubes is sufficient to obtain good tribological properties. A structural modification of nanoparticles during friction was clearly evidenced by analytical TEM. In the case of nanotubes, flake-like wear debris made of amorphous carbon have been observed .