For some years, reports have been published on adding solid lubricant powder to oil to improve the tribological properties of the latter, but the results have not been satisfactory. In this paper, we describe the preparation of CaCO3 nanoparticles in a microemulsion consisting of sodium dodecyl-sulphate (SDS)/isopentanol/cyclohexane/water, and assessment of the tribological behaviour of CaCO3 nanoparticles as additives for lubricating oils. The CaCO3 nanoparticles were characterised by transmission electron microscopy (TEM), and their tribological performance was tested in a four-ball machine; the rubbing surface was analysed with X-ray photoelectron microscopy (XPS). The results indicate that the size of CaCO3 nanoparticles increased with the concentration of aqueous reactant, and that CaCO3 nanoparticles exhibited good load-carrying capacity, antiwear and friction-reducing properties. The tribological properties of lubricating oils could be improved significantly by dispersing CaCO3 nanoparticles in 500SN base oil containing dispersants such as polyisobutene-butanediimide (T154), calcium alkylsulphonate (T101) and methyl-tricaprylamine chloride (aliquat 336). The improvements in friction and wear were concluded to be due to the formation of a film containing CaCO3 and CaO in the rubbing region, and the presence of nanoparticles, which may act in the same way as ball bearings, to facilitate sliding.