Abstract: In this study, three kinds of calcite calcium carbonate nanoparticles (CCNP) with different average diameters were successfully fabricated via the carbonation method. The morphologies and phase compositions of the CCNP were determined via transmission electron microscopy and X-ray diffraction. The effects of the concentration and size of CCNP on the tribological properties and rheological behaviors of grease were investigated to reveal the lubrication mechanism via UMT-2MT friction tester and HAAKE RS6000 (Germany) Rheometer. The results show that the tribological performances can be improved significantly by addition of CCNP. There existed optimum CCNP concentration and size, where the grease can exhibit simul-taneously optimal anti-wear and friction-reducing properties. Meanwhile, the rheological performance of greases greatly influenced their final tribological properties. The excessive concentration of CCNP provided negative effects on rheological performances and further affected the final tribological properties, especially at rigorous test conditions. In addition, X-ray photoelectron spectroscopy show that tribofilms with the similar composition of calcium carbonate,calcium oxide,iron oxides and other organic compounds were found on the contact area. The selectivity of tribological performances of CCNP for the test condition may be due to the difference of the mechanical properties, which depended on the different sizes of CCNP.