Abstract: Nano-lubricating oils were prepared by adding equal mass fraction (1%, 2%, 3%) of SnO₂ nanoparticles and oleic acid into poly-α-olefin (PAO₆) base oil. The tribological performance of the nano-lubricating oil in steel ball-brass block tribo-pairs was studied by reciprocating sliding friction-wear tests and compared with that of non-nano-lubricating oils, and the frictional reduction and anti-wear mechanism was also discussed. The results show that when lubricating in the nano-lubricating oil with no less than 2mass% SnO₂ nanoparticles, the average friction coefficient of the tribo-pair and the width and depth of the wear scar on the brass surface were all lower than those with the corresponding lubricating oil without SnO₂ nanoparticles. When the mass fraction of SnO₂ nanoparticles was 2%, the nano-lubricating oil had the best frictional reduction and anti-wear performance; comparing with those lubricated in PAO₆ base oil, the average friction coefficient and the width and depth of wear scars were reduced by 31%, 42%, 50%, respectively. The excellent frictional reduction and anti-wear properties of the nano-lubricating oil were attributed to the lubricating film formed by oleic acid and the protective layer formed by SnO₂ nanoparticles.