Abstract: Nano-mullite with different mass fractions (0.01%, 0.03%, 0.05%, 0.10%, 0.40%) was added to polyurea grease. The influence of nano-mullite on tribological properties of polyurea grease was evaluated with a four-ball friction tester, and the anti-friction mechanism of nano-mullite was studied. The results show that with the increase of nano-mullite addition amount in polyurea grease, the stable friction coefficient of friction pair and wear mark width decreased first and then increased, and reached the minimum values when the mass fraction of nano-mullite was 0.03%, which were 0.058 7, 0.64 mm and were reduced by 9.3% and 11.1% than those under lubrication of base grease without mullite addition, respectively. The excellent tribological performance of polyurea grease added with nano-mullite was attributed to the formation of a lubricating film during the friction process, which consisted of polyurea grease, compounds containing C—O bonds, nitrogen oxides, and mullite. The SiO₂ in the mullite helped to reduce contact pressure and change the friction mode of the friction pair to rolling friction, while the Al₂O₃ filled the pits and depressions on the surface of the friction pair, promoting the formation of the lubricating film. The synergistic effect of these two components resulted in good lubrication performance of polyurea grease.