Abstract: An oleic acid surface -modified calcium fluoride ( CaF₂ : ) nanoparticle ( OA -CaF₂ : ) with average diameter of 23 nm was chemically synthesized using an extraction method The morphologies and crystalline structures of the powder have been characterized by transmission electron microscopy ( TEM ) and X–ray di ffraction ( XRD ) , while the chemical composition and further information on the structure of the nanoparticles have been characterized by FT -IR The results showed that surface of CaF₂ nanoparticles had been modified by oleic acid successfully The tribological properties of the prepared OA -CaF₂ nanoparticles as an additive in lithium grease were evaluated on a four -ball tester The results indicated that nanoparticles exhibited excellent anti -wear and extreme pressure ( EP ) properties It 15 also found that OA -CaF₂ : nanoparticles had good friction -reducing ability at high loads SEM , EDS and XPS analyses indicated that a boundary film mainly composed of CaF₂ , CaO , Fe oxide and some adsorbed organic compounds was formed during the friction process These contributed to the good friction -reducing and good antiwear ability of OA -CaF₂ nanoparticles as lithium grease additive